In order to change formation in the game, you should go to the Team menu. Under "Formation" tab, you can drag and drop a new player, or grab, hold and move the ones on the field. Also, you can use the folder icon in the middle left side to save up to 3 formations.

Also, You can select the formation you want by clicking the formation button in the top left corner, or you can also drag and drop players yourself to change the formation.