You are given the option to select your fire mode when you first sign in on mobile. After that you can change your fire mode from within the HUD Layout Tool accessible from the menu. Details instructions are below:
1. Tap the menu icon.
2. Tap HUD Layout Tool.
3. Tap the arrow in the top left corner of the screen to expand the side menu.
4. Tap Change Fire Mode.
5. Tap the fire method you’d like to use.
6. Tap Select to confirm your fire mode. You successfully changed your fire mode!