For rounded nipples I would just take the tyre, tube and rim tape off and extract the nipple using a flat head screwdriver and then replace the nipple, however if it is a machine built wheel there is a chance that the nipple would look a mess in the rim. I recently took one apart and it was shocking just how badly it was put together. If that is the case then look at getting a replacement spoke as you might have to extract it by cutting the spoke. If you go down the destructive route, make sure that you wear eye protection when you do it as the nipple end can fly off with some force.