Changing the map during a multiplayer game relies on a vote based system. Each player can, at any time, suggest a different map than the one currently playing. When this happens the other players have to vote for that map or suggest another one. When a map gets the votes of the majority it is selected and the game is restarted using that map and it's specific gameplay mode. Each player's vote/choice will be displayed in the top left hand corner of the screen in the following format : Player X suggests Gametype Y on Map Z (select map to vote).

To suggest or vote for a map, from the Multiplayer menu go to the 'Map selection' area that only shows when connected to a game, choose the desired game type, or the suggested map's game type. A list of all avaialable maps in that category will show up allowing you to easily select another one or vote for the one that was suggested.