The first step is to back up your wallet.dat. The Bitcoin Wiki has information on finding your data directory, depending on your operating system:
https://en.bitcoin.it/wiki/Securing_your_wallet#Windows_2Then download the latest version and run the installer. If everything goes correctly, you won't have to do anything else - your wallet will still be intact. If something goes wrong, you'll have to import your wallet from your backup. As long as you have a good backup, though, there's very little risk.