Or put the LiveCD into the drive and boot from that. Click "Install", go through all the screens and when you come to partitioning mark the / partition for formatting, mark the /home partition to be used but not formatted and install your new Mint. Do not mark anything else (Windows) or you will wipe it.
There is yet another way, similar to the one suggested in post 2, that lets the machine do all the work meaning you don't have to edit any files. After Mint 12 is released Update Manager (if you have it installed as I know Mint have their own Updating program) will tell you a new version is out and will give you the option to upgrade to it.
The reason I am suggesting the first method is that Ubuntu and derivatives of which Mint (number versions) is one are notorious for stuffing up upgrades. If you do a clean install you still have your old settings in your /home partition but you don't have any "old" crap hanging around in / so you minimise the chances of problems when you reboot.