What kind of gun and what kind of sights --- many non-gunsmiths, let's call them technicians at stores and ranges can push one set of service sights out and another in with no problem using a sight pusher that's a little expensive to buy for 1 job at home, but then some can butcher even this simple task.
If you are talking a 1911 or other slide that needs to be milled to take the new sight than it's a job for a pro Novack's is quite good in terms of service and their sights are top notch, though there are lots that are also good or lots of smiths putting on novacks.
More info might help up give better overall advise.