With a Unitronics PLC, you can use the toggle function -
SB41 MB0
I------I P I----------(X)
Every ON transition of Key 1 on the integrated keypad toggles the state of MB 0, or Output 0, or whatever bit you like.
Of course, your instructor probably wants you to do this the old way, with latching bits and multiple outputs, etc, etc. It will be good practice for you. The whole "learn to drive a stick shift" thing.