Turn off the meter, open the back, pull the shutter curtain to the right side and release it. The curtain should have enough tension to release the mirror and stop down mechanism.
If this solves the problem it's just needing an additional click or two on the closing curtain tension adjustment. If you remove the bottom cover, there are two gears near the middle of the camera. They will have a set screw probably sealed in place with nail polish. Clean off the polish, loosen the screw & tighten the screw beneath the set screw. A partial turn should do it, try the camera. tighten the set screw, seal it if you want