This might be a very simple question, but I can't seem to find the answer around! I ran useDevel() for a package I'm developing and now I'd like revert back to not using the development packages, what's the best way to go about this?

Use useDevel(FALSE). This will install the release version of the BiocInstaller package, and downgrade (re-install) packages that are at their devel version. Verify that the installation is correct with BiocInstaller::biocValid(). More information is available on the corresponding help pages.