Just uninstall and reinstall SQL Server, and you'll get another 6 months of evaluation of SQL Server.
When you uninstall it, use the add/remove programs control panel item.
Don't delete the C:\Program Files\Microsoft SQL Server directory because this will likely delete the database .mdf and .ldf data and log files - indeed back these up before doing anything.
Also when you re-install SQL Server, the databases won't initally appear in the "databases" folder in SQL Management Studio. To get them to appear, right-click the "databases" folder and select Tools - attach - and select the .mdf file for the database you want to attach.
Bear in mind that if you are making money out of your SQL Server, you should really pay for a licence to use it.