The Control Center on iPhone, which is accessed by swiping up from the bottom of your screen, gives you instant access to many useful controls and can be customized to fit your needs. To do this, go to Settings > Control Center > Customize Controls. The controls that currently appear are listed at the top under Include. You can tap the red minus sign to remove one, or tap and hold to drag them around and change the order. Available controls that are not active are listed under More Controls. Tap the green plus sign to add more.