Open the Settings app
Tap Sounds & Haptics
Under Ringer and Alerts, drag the volume slider left or right
Turn off Change with Buttons if you want the alarm volume to stay fixed
Make sure your alarm sound is set in the Clock app
Open the Clock app
Tap Alarm
Tap Edit
Tap the alarm you want to change
Tap Sound
Select a louder or quieter alarm tone
Save the alarm settings