When I had to teach folks to drive a "crash" gearbox, the key saying was revs up to change down and revs down to change up. It is a matter of synchronising the gears into mesh. If you think of the engine speed just before you change up the gearbox, that is the engine revs you need to align the meshing gears (as you rev in neutral with the clutch out) to change back down the box. Sounds complicated but it becomes second nature once you get into the habit.
https://www.youtube.com/watch?v=KlS6DF3NjJE
