If you have SQL Server 2000, SQL Server 2005 or SQL Server 2008 Feb CTP on your PC and then if you want to upgrade to SQL Server 2008 RC0, on the UI side, you only can see the link which can let you upgrade from SQL Server 2000 or 2005 to SQL Server 2008 RC0.
How do you upgrade from SQL Server Feb CTP to SQL Server 2008? I would like to say that you can click the above link to try if the upgrade is working. I believe this link also supports Build-to-Build upgrade.
If you don’t want to use UI to do the upgrade, you can also use the command line to do the upgrade as follows:
setup.exe /ACTION=Upgrade /INSTANCENAME=yourInstancename
After entering the command line, you will know how to upgrade to SQL Server 2008 RC0.