When you go to T5B, get in the lift rather than the escalator. The lift has buttons for the transit one floor down and a walking route two floors down. Take the walking route. Remember where it comes out, as I have no idea. On your way back to T5A, use the door that you came out of to get back to the bottom floor and retrace your steps.

Alternatively, assuming there is a lift at the other end from the transit to departures, check the buttons in it. It might take you all the way down too. If I go from T5B tomorrow, I'll try to remember to check.