How do I zoom in and out with my weapon?

To zoom in/out, you can tap the zoom button on the left-hand side of the screen. Continue to tap this button to go through the different zoom levels.

You can also use a pinch motion with your fingers to zoom in and the opposite to zoom back out (this option can be turned off in the "Settings" menu).

Note that you can customize the control layout in the “Settings” menu which will allow you to move the buttons wherever is suitable for you.