[Haskell-beginners] How to build a package?

I used cabal unpack to get the source of a package to read. Now I want to make a change. How do I tell Haskell Platform to make that (the same way cabal knows how to) and do so with the provided files rather than downloading into temp. I got a package to work before using Leksah, but I'd like to do it directly since the cabal file is present and works just fine. A related issue: what should I do about the local copy of a module name that is also installed? Can I make my project use that instead, without having to globally change the name in all the files?