Ubuntu 16.04 have support until April 2021. So in 2021 will i have to reinstall whole system or will be asked to update system by"system update like on windows" without formating hdd and losing all my preferences? Like every 5 years have to reinstall everything? Sorry to ask silly questions. I'm newcomer to Linux and I just want to use pc for daily things and don't want using terminal at all. Thank you
LTS releases occur every 2 years, so from 16.04, you will have opportunities to upgrade directly to 18.04 in April 2018, and then to 20.04 in April 2020. It will be generally expected that you would have upgraded to a newer release by then. You will be able to upgrade to 18.04 still at that point though.
You can also re-install a fresh new version at that time if you need to (you may very well have a new PC by then), but it will not be necessary. However, if you do not upgrade by that point, you will no longer be running a supported system, and you may become susceptible to security problems if new ones are discovered in the versions included in the no-longer-supported version of Ubuntu.
You can stay on an LTS release for 5 years and continue to receive updates. After 5 years you will not be forced to upgrade. If you don't, you'll simply stop receiving updates, and thus your system can be considered to be unsupported.
Your options at that point would be
Continue running an unsupported release. Security issues may not be fixed and you can't receive support from Ubuntu (or this site). It is considered irresponsible, even harmful, to run a Linux distribution that is unsupported due to the potential for security problems to impact others.
Upgrade to the immediate next LTS release using an in-place upgrade, in your case 18.04. That will then ensure you are supported until April 2023. You can then upgrade immediately to 20.04, or wait until April 2023 to do so.
Upgrade to any release by reinstalling.
You can continue using 16.04 even after 2021. There is no compulsory update or upgrade you have to perform. If you are happy with the setup, you can use 16.04 up to 2030 or as long as it works.
As mentioned, LTS releases are launched every 2 years. 16.04 in 04/16, 18.04 in 04/18 and so on. Most LTS users tend to upgrade to the next LTS as and when it arrives. But again, that is just a choice. You are not forced to switch from 16.04 at all. Plus, you get updates for it till 2021 as well.
If you look at
how to guides, most of them support up to 14.04 at the moment. Most software vendors assume that you are with the latest LTS. Once you stay on an LTS that's beyond support,
You might not be able to install the latest version of many software
Lack of security updates might expose you to security concerns
Other than that, your PC should work just fine, even in 2040. :)