On the right of the phone, there is one long button and one smaller button. The long button is the volume button. When the phone display is on, press the volume button to display the volume menu on the right of the screen. Tap on the gear icon at the bottom of this menu to access the Sound menu. The third line item is to control the "Ring volume" - slide to the right to increase, slide to the left to decrease.
Note: if you press the volume button to bring the volume to the lowest setting, the phone will be in vibrate mode. When in vibrate mode, your phone will vibrate when there are incoming calls and texts and other notifications.