Any old school clock and watch repair shop can easily do this work, but so can you. There are a number of threads on this topic (
https://forums.pelicanparts.com/search.php?searchid=23734814) (this one should work:
https://forums.pelicanparts.com/porsche-911-technical-forum/913721-vdo-clock-repair-incl-pictures.html), so you should be able to find one for working on the quartz movement that yours should be.
Usually, what happens is the capacitors stop holding charge. I replaced both of the caps in mine and it works perfectly now. You'll need to have a modicum of soldering experience, but I'm more than sure there's a youtube video to get you through that if you need.