Upgrade from a trial version of Office SharePoint Server 2007

Use the following procedure to convert a license type and enter the product key.
Convert a license type and enter the product key

1.In Central Administration, on the top link bar, click Operations.
2.On the Operations page, in the Upgrade and Migration section, click Convert license type.
3.On the Convert License Type page, in the Enter the Product Key box, type the new product key.

Reffer: http://technet.microsoft.com/en-us/library/cc263204.aspx