To go back downstairs to immigration, just go down any of the escalators that are coming up (okay, take the down escalator next to the one coming up) from arrivals. These are spaced throughout the terminal.

Or just walk backwards through the main security that leads up from arrivals. Just tell the guards that you want to go to immigration. Then, take the elevator down (no down escalators here). This is in the center of the terminal, on the opposite side to the fountain and the large elevators.

It's very easy.

On the other hand, I once saw a gentleman walking back up the long escalator from arrivals down to immigration. I wouldn't recommend that...