To make substitutions, click on the icon of the benched player you wish to substitute in, hold down the button and "drag" the player on to the pitch.
Players you can validly substitute your benched player for will remain visible, while invalid options will become semi-transparent. Finish "dragging" the player over the player you wish to substitute them for and release the button. You should then see the players swap round. Repeat the process if you wish to make more than one substitution. When you are finished, press the Save Changes button beneath the pitch or press Reset if you wish to start from the beginning.
For more information on team changes and game rules please visit the Game Guide section.