What exactly are you looking for? Is it a method to update the contains tab of an assembly you have checked out and modified? Or a way to update the contains tab of an assembly that is checked in, but some of the components have been modified?
Either way I don't think there's much that can be done. For the checked out assembly there is no reference in the database to the new version until it is checked in. I had an assembly at version 6 that I checked out and modified, then added the references into the xrefs table and xrefconfiguration table manually (not supported at all, but gives you an idea of how an API program would work). The BOM tab and the Contains tab still only showed the checked in versions, which means it's driven off the revisions table. If I then manually add a version to the revisions table (again bad idea, only good for testing) I can see the references I manually created as version 7, but when I then check in the assembly it becomes version 8 which is essentially the exact same result as checking the assembly in and back out. Even using unsupported methods I don't see any way to make that work.