I apply for minor changes to our aircraft all the time. The request normally goes to EASA for inital approval. When this comes through you send your technical package to the niominated Authority (Usually your own NAA). Then depending on the technical content of the package, you could be waiting for between 4 weeks and 4 months. PM if you more info.
You can go to a PART 21 authorised company who will do the minor dhange and authorisation for you, but you're talking bigger bucks then!!!