How to update map data for games that have already

Hi, I’m currently planning to launch a game that uses one map.I’m serving one map on the server and the client files have just one map as well. But I want several maps. But every time I add a map, it’s very inefficient to re-download all the game data, so I just want to add a map file. I’m trying to get the player to move to that map after adding a map file, do you know how to do this?