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
Use the volume buttons to adjust the ringer and alert volume if Change with Buttons is enabled
Open Clock app
Tap Alarm
Tap Edit
Select the alarm
Tap Sound
Choose a louder or quieter alarm tone
Save the alarm changes