You need to construct tiny ladders for them to move up. Instructions below*

Or you could just switch things up. Move the bottom tray to the top if you think it's 'done', and leave top off so there is light (ideally bright), they don't like light so should retreat down below. Probably it will dry out some up there and they tend to prefer moister (within reason).

Note, I've never used one of these three bin systems, just making a suggestion. And of course if the worms are still working down there, they're happy and you could leave them alone. Also, you'll probably never get all the worms to leave there, that's fine as long as the other bins have some worms in there.

*Worm ladder Instructions: just kidding.