Open the Clock app
Tap Alarm
Tap Edit
Select the alarm you want to change
Tap Sound
Choose a louder alarm tone
Go to Settings
Tap Sounds & Haptics
Drag the Ringer and Alerts slider to the right
Turn off Change with Buttons if you want the volume to stay fixed
Make sure Silent Mode is off if needed
Check that Do Not Disturb or Focus is not muting alerts
Test the alarm volume with a short alarm time
Restart the iPhone if the volume still seems low