Open the Clock app
Tap Alarm
Tap Edit
Select the alarm you want to change
Tap Sound
Adjust the Ringer and Alerts volume in Settings
Go to Settings
Tap Sounds & Haptics
Drag the Ringer and Alerts slider to the desired volume
Turn on Change with Buttons if you want to use the volume buttons
Use the volume buttons to adjust the alarm volume if enabled
Save the alarm settings by tapping Back or Done