Thats up to you. Some people have complained that it has warning about copyright violation that can't be removed. Not necessarily a bad thing but maybe a nusiance.
If you do decide to update to the newer version, save the installer for 4.12.6. Should be found in c:\Program Files\LimeWireWin.exe. Move it somewhere else for safe keeping, like in your "My Documents" folder. Just incase you don't like the new version you can go back to the old one. The reason for moving it is because when you install a new version it removes the old one.
You update it by d/loading and installing the newest version. Get it here:
http://www.limewire.com/english/content/download.shtml