you can use the tutorial at XDA, its like 3 mouse clicks. Use the toolkit
http://forum.xda-developers.com/showthread.php?t=1392310
and then the tutorial is this
http://leemn.wordpress.com/2012/02/01/galaxy-nexus-upgrade-to-official-roms-yakju-build/ scroll down toi see the tutorial using the toolkit. Basically you are unlocking the bootloader, then download the image you want, there is an option in the toolkit, and then you just click to flash the image, the rest is taken care by the toolkit.
Remember that you need to have a backup of your sd, pictures, mp3's and stuff because you will loose everything.
If you find the courage to proceed
, i suggest the latest 4.2.1 JOP40D or JZO54K 4.1.2 version. Here you can find all the images for the Gnex
https://developers.google.com/android/nexus/images