Hopefully the coax that is there runs back to a common or central point. You want to run the coax from the dish to that common point. There, you will put in the power inserter for the SWM LNB (in the dish), and a splitter. Connect the coax that goes to the rooms to the outputs of the splitter.
You won't be able to use the same coax for anything related to cable (tv or internet), so be sure to figure out everything you have and how it runs before disconnecting anything that you might want to keep.
You won't be able to use the same coax for anything related to cable (tv or internet), so be sure to figure out everything you have and how it runs before disconnecting anything that you might want to keep.