如何从VCL重新编译特定单元？

I want to apply a fix from QC to a Delphi 2009 unit (DBClient as it happens). I know I need to copy the unit to another directory and make the change to the copy. How do I then get Delphi to compile that unit and use it in favour of the DCU that already exists?I want to apply a fix from QC to a Delphi 2009