Welcome to the board!
Sounds like you have the keyboard out of the synth already.
Find a small (1/8?) Allen wrench that will just go down into the hole in the keyspring. Press it doen to unlock the lower end of the spring from the black plastic "comb" under the back of the keys. Slowly release the pressure and remve the spring.
Next, take a small flat screwdriver and CAREFULY use it to press the tab in the slot at the back of the key to release it from the "hinge".
DONT BREAK THEM!!!!! Use just a little pressure and only a fraction of an inch motion . Lift the key out. NOTE: to change a black key, take the white key on both sides first.
On the new key, get the front "hook" onto the rubber bushings, the press down on the back end to snap in back into the hinge. Place the spring into the funnel-shaped hole on the back of the key, and use the Allen wrench to stretch the spring down to catch on the black comb.
"You will live to see man-made horrors beyond your comprehension."