You will need good screwdrivers, a pair of tweezers and a steady hand.
So here we go:
Unfold the camera. No need to unfold the viewfinder.
Remove the 2 screws that hold the "Welta" nameplate.
One of my Weltas had shims beween the nameplate and the viewfinder.
On the backside of the viewfinder there are 2 more screws in locations similar to the ones in the front.
Remove them.
Lift off the viewfinder. It might be stuck, but it is possible to just lift it. Be cautious, the focusing screen is not fixed and might fall down. Take off the screen.
The part that is coming off the camera.
You now have access to the mirror (this one has suffered a lot).The mirror is held in place by 4 small metal springs that are screwed on. Remove the 2 on top. In my cameras all springs were screwed from the backside. They are nevertheless accessible, as the mirror holder can be moved forward in it's hinges. You will need a very short screwdriver to unscrew them and it's tricky not to lose them. You can rescrew them from the upper side which is much easier.
Slide out the mirror from under the 2 bottom springs. Clean the inside of housing with the aid of a brush. This is the unique opportunity to clean the back of the viewing lens as well. Slide in the new mirror. When you install the new mirror, make sure the side with the protective foil is up and forward (on the picture it's the wrong side up). Rescrew the 2 upper springs. Clean the focussing screen, put it back, re-install the viewfinder and re-screw it. The effect of a new mirror will be amazing. Good luck!
If you want to further improve the viewfinder, you can put black tape on the gap between the frame and the pop-up shields. You need tape about 15 mm wide, 2 strips of 47mm and one of 60mm. The tape should not stick to the folding area and has to be put on when the shields are folded down. To acheive this, put a strip of thin plastic foil about 8mm wide on the middle of the sticky side (see photos). Not pretty, but helps a lot.
