Open the Settings app on your Apple Watch
Tap Sounds & Haptics
Tap the volume controls or turn the Digital Crown to increase the volume
If needed, adjust the volume while audio is playing by turning the Digital Crown
On iPhone, open the Watch app
Tap Sounds & Haptics
Drag the Ringer and Alerts volume slider to the right
Make sure Silent Mode is off if you want louder alerts
Check that Do Not Disturb or Focus is not reducing notifications
If using headphones, increase the volume on the connected device as well