I did this recently from A.
Just take the lift or escalator back down as if going back to train. When you get down to same level as train, approach one of the staff in a Dubai Airport Authority t-shirt and explain you are in transit but just received a call and need to pop into DXB briefly.
They will escort you to a lift, open it with a key, and give you directions to immigration once they have sent you on your way.
It seems this is a frequent request as the staff who helped me did not bat an eyelid.