1. Goto “help > check for updates” in eclipse menu :
2. Let eclipse search for updates, until it comes with a dialog like this. Select “Force.com IDE” and hit “Next” button, as shown below:
3. Hit “Next” on the next screen :
4 : Accept the license terms and finish as usual :)
5. Let eclipse download the updates, once done you will get a notification to restart Eclipse. Restart it and you are all set to use Winter’12 IDE.
After restart you will notice this classic popup asking for Force.com project upgrades.