UPDATE Query

Hi, I have (amogst others) three tables in my database named Bookings, User, and Representative. A User and a Representative are different types of user, however I now want to merge these tables into one - User. When a Booking is made, the User_ID and the Rep_ID are stored in the Booking table. I have now merged the two tables (User and Representative), how can I update the Rep_ID column in Bookings so that it refers to the User_ID for each Representative in the User table? Thanks for your help