normally you just need to intall the new bitcoin version over the old one and all will be ok. to backup you wallet, just click on the menu "File -> Backup wallet..."
It would be VERY WISE to backup your wallet before upgrading just in case something goes wrong.
As a matter of fact, it would be VERY WISE to backup your wallet on a regular basis (perhaps every 25 transactions or so) and keep at least the 3 most recent copies stored somewhere off your computer. If your computer crashes or is destroyed, those coins could become permanently lost if you don't have a good backup.