Hi, how can i change the office after i rejected the offer to change it? Where do i have to click in the garage?! 
You will be able to change your office again when you release a new game. It was so for me 
When you’re given a choice like that (changing offices, building the R&D and Hardware labs, etc) and you say ‘Not Yet’, then the question will usually crop up after about 3 months (or after you’ve developed your next game). The exact timing varies a little, but basically just carry on as normal, and the message will return in time.