A. Check the readme.txt that came with the sp. Some come with un-install instructions - in which case follow them.
If there are no instructions then just replace the updated .exe's and .dll's with the previous ones. This won't back out any of the SQL script changes that would have been run as part of the servicepack, but I don't know of any issues that require these being backed out.
For 6.5 SP5a back to SP4 there is a kb article at http://support.microsoft.com/support/kb/articles/Q216/4/21.ASP