By default, when you press the volume key on LG G5 while the alarm is going off, it snoozes the alarm. But to use volume key to dismiss alarm on LG G5, you need to do as follows:
1. Open the Clock app on your LG G5 (either from the App Drawer or the Home Screen)
2. Make sure the Alarm tab is active, and then hit the 3-dot icon on the top right
3. Go to the Alarm's Settings.
4. Touch to open the Volume Key Behavior option
5. Select the Dismiss option (Other available options are: None, Snooze, and Silence)