To switch between your Primary and Secondary weapons during combat, an appropriate Secondary weapon must first be equipped. To do so, open the Store and click on "Heavy Guns" or "Tesla Rifles" (for Short Range or Long Range combat, respectively), then either choose an owned weapon or buy a new one, and click 'Equip'. Once you're in battle, if an appropriate Secondary weapon for that combat range has been equipped, a "Switch Weapon" button will be available (see below, circled in red). Click on it to switch between your Primary and Secondary weapons, and vice versa."