Since snapshots/prereleases don't require any sort of effort, you're going to have to move a few files around.  There's a folder named "Snapshot" in the zip file, go into it, copy all the files somewhere on your computer, go to the texture pack zip file, and then copy the files you copied onto your computer into the texture pack zip file directly.  If it asks you to replace the files, make sure you say yes to any and all of those options (it usually doesn't).  Now the texture pack is ready for the newest Snapshot/prerelease.  Have fun!  :D

Note: If there are no files in the snapshot folder, then I have not updated yet or moving files around isn't required.