21.04 is not an LTS release and cannot be turned into one (short of upgrading it into one in the future).
Currently there are 2 active LTS releases, so if you want an LTS today, these are your choices: 18.04 and 20.04. (I offer: out of which, at this point, due to it being more recent and offering 2 more years of support, 20.04 could be the more sensible choice.)
How to interpret LTS
An LTS version means: "Long Term Support" version.
It works such way that the collective of contributors decide (and pledge) that they will keep putting work into a given release (identified by the version number), to keep it alive and working flawlessly for a longer time.
In contrast, non-LTS releases mean that the contributors decide / agree to stop putting any more effort into those given versions, after a relatively short period of time. After that short support period expires, developers become free to pour their efforts into newer versions, and to tend to the ongoing needs of currently active LTS releases.
For this reason, a given version number does not have an additional LTS variant, neither a switch to enable LTS-ness.
The answer, instead, is simply: have the contributors pledged to support this given version for a longer time? The answer is a simple yes or no.
(Note: thanks to substantial feedback from @guiverc and @ThomasWard, this section now contains updated information.)
Can such a designation as
Ubuntu 20.04 LTS be considered redundant?
Well, yes and no.
In the past 10 years, it got established that every even year's April release (e.g.
20.04 — in which the first number designates the year, and
04, the fourth month, April) was an LTS release.
Insiders and experienced users then can recognize these releases as LTS'es, without the explicit
How about the future though?
Well, in my original answer I offered that any version number matching this pattern (even in the future) would be rightfully expectable to be an LTS release (solely on the grounds of adhering to the convention). Turns out, I was wrong.
Thomas, in his answer in this thread (and in the appended clarification), revealed that however stable this ongoing pattern may come across, one should not take it as guaranteed that next releases strictly adhere to the pattern. Merely expecting it is okay though, I take it.
In case of doubt, one should be able to look up the corresponding Ubuntu flavor's documentation on releases, to check on this. Being the user of stock Ubuntu, I am familiar only with its corresponding release cycle documentation, but right now I don't know where to look up the one for Lubuntu. (Could someone suggest it please?)
(Regarding this linked document, it's also important to note that while the main Ubuntu project now offers 5 years of standard maintenance support on LTS releases, other Ubuntu flavors (including Lubuntu), as a rule of thumb, offer only 3 years.) (If interested in specific details of further variants of Ubuntu, you are advised to look those up (probably on those projects' websites).)
How to get to an LTS from Lubuntu 21.04
If you want to switch to an LTS release from your current 21.04 installation, your choices are:
- Re-install from the 20.04 iso; this would involve destroying your current 21.04 installation, so, whether to opt for this or not, you will decide based on how much effort you have already invested into setting up your current 21.04 installation.
- You can also keep your current 21.04, and go along with upcoming minor upgrades until you arrive at an LTS release. If you manage to do that upgrade, you will become "covered", and will have met your goal then. According to the above introduced pattern, the next LTS could be 22.04, getting released in April 2022.
- Now, in this case you would have to plan with at least one intermediate upgrade to yet another non-LTS, 21.10. The reasons that make this step necessary are the following:
- since 21.04 is not an LTS release, it will lose all support after a mere 9 months (according to this info here) (same as in regular Ubuntu), 3 months short before 22.04 gets released. So, to keep you covered with support (security patches) you will need to bridge over this 3 months gap by using the next minor release.
- additionally (as I have learnt from @guiverc), the 21.04 system will never be "made aware" of the existence of the future 22.04 release, so it will not be able to perform the upgrade to 22.04. 21.04 will get enabled to upgrade only to 21.10. It will be, in turn, the 21.10 release that will get equipped with the necessary faculties (also called "upgrade path") to upgrade to 22.04.
Some further hints to help with making a choice
Non-LTS versions always tend to have fresher software ticking in them than LTS'es. This means a more recent kernel (wich includes some drivers for newer hardware), and also, more recent versions of user-space programs being available through the standard repositories. That's their main attractive side, I believe. (Btw. some similar advantages might be achieved on LTS'es too, but only at the cost of going through extra hoops.)
Now, to the strong side of LTS'es:
(L)Ubuntu (just like other Linux distributions, I presume) offers itself easily for substantive amount of tweaking and customizations. However, depending on how far you go with these, they may start to have an impact on how smooth results you may expect from upgrades. (To put it bluntly, it's about how much your installation appears to get demolished after carrying out the upgrade.) (People, in such cases come to this site and ask for hints on how to restore law and order on their machines. But it's already a headache, and extra efforts then (to put it mildly).)
So, if you are a heavy tweaker (either because — being a beginner — you plan on to try out a lot of things, or because you are someone with a strong appetite for special configurations), I would rather point you to an LTS release, which allows you to ride it out for longer, without having to worry about how any version upgrades would go, simply because you don't have to do upgrades. Your system just keeps steaming ahead on plain software updates.
Additionally — in a use-case avoiding upgrades altogether — if I ride out an LTS for 4+ years with relative convenience, then, when its support finally exprires, I don't mind at all to do a fresh install of the new LTS, and rebuild everything again. (Oops, I mean so many years with the main Ubuntu release.) (On Lubuntu, it could be somewhere between 2-3 years in optimal case, and now, considering 20.04 already being a year old, that much less.)
Sooo, yeah: you do your math, and decide from all this :)