How do I upgrade release from 11.04 to 12.04?

You have two choices you can either backup and do a clean install or upgrade.

In either case backup first because updates can go wrong.

Personally I would go for a clean install. You may want to consider Xubuntu if you have an old/slow machine

Downloads are available here. Make sure you check the MD5sum to ensure the download went OK.

See here: https://help.ubuntu.com/community/HowToMD5SUM

I suggest you try each of live CD / USB first, Make sure everything works then when you have chosen the one you like do a full install. You may find Ubuntu is too slow depending on your hardware.

Ubuntu:
http://releases.ubuntu.com/12.04/

Xubuntu:
http://cdimage.ubuntu.com/xubuntu/releases/12.04/release/

If you want to upgrade instead you will need to go from 11.04 to 11.10 and then 12.04

Open a terminal (CTRL+ALT+T) and enter

sudo apt-get dist-upgrade

If this goes smoothly reboot and login then

Open a terminal (CTRL+ALT+T) and enter

sudo apt-get dist-upgrade

If this goes smoothly job done mark this question as solved. If you have problems for either upgrade reply back here with the details and we will try to help further

I'd go for a clean install personally, restore user data from your backups

I agree. A clean install will be faster and probably give you less issues along the way.