How do I upgrade my Ubuntu to 16.04 ? It is now 12.04. If I upgrade the version will it delete all my installed apps ? Kindly help.
To upgrade from 12.04 to 16.04 you have to upgrade first to 14.04 and then to 16.04. It can be done by UpdateManager. But I would recomend a clean install. It would probably take less time than double upgrade. Backup your date before any of this actions.
You can upgrade Ubuntu from live USB key with 16.04. I thik it should offer you an option to upgrade from older versions. But I thing the programs you have installed will be lost.
In addition to the other answers, an upgrade should leave your applications in place, although they themselves may need to be upgraded to work properly on the newer version: for example, the OS upgrade may bump the version of mysql, which could have implications for an installed application. Bear in mind that third-party PPAs will be disabled during any upgrade, so you'd need to check the sources files for Precise (vs Xenial) or edit the sources via the appropriate GUI tools (e.g. Synaptic).
It's actually good practice to not only do a clean installation but also use that process to document what and how you added afterwards. It makes migration and/or rebuilds much easier if you have decent notes, and saves the "oh, hang on, what was that command/where did that script go?" sensation much later on.
(apologies for the separate answer, but I don't yet have the reputation to comment)
You can upgrade from LTS to LTS with the command:
You would have to upgrade from 12.04 to 14.04 to 16.04.
I recommend not doing this. A clean installation is best practice as it keeps your system fresh and generally improves performance. It is also far quicker.
Sadly, you cannot keep your programs, but they can be reinstalled (and should work better, given the upgrade). Backup your important data and port it after you have upgraded.
I'd recommend upgrading only to 14.04 for the time being. There have been many-many reports of problems with both upgrades and clean installs of 16.04 -- including my main desktop system, which I had to recover from backup after upgrading 14.04 to 16.04 resulted in failure to load the desktop. You can remain supported on 14.04 until 2019, so you can ride that version until either 16.04 has its bugs ironed out, or 18.04 is ready.