I’ve done some touch-ups in the northern germany refuge yesterday. One thing was changing the 20 year old window handles for new, lockable ones. It’s really easy, also for the inexperienced and here’s how:
First bring the old handle in the “open” position and flip the cover sideways 90 degrees. Unlock the new handle and do the same. Then unscrew the old handle.
Insert the new handle in the “open” position into the locking mechanism and tighten the screws. Flip the cover over the screws and check if everything works smoothly. You’re done.