Once you've mastered breathing without a mask and clearing water from a flooded mask, you'll be able to remove and replace the mask while underwater. You'll use this skill in the event your mask slips off, or if you need to remove the mask to correct its fit. To replace your mask underwater:

1. Hold the mask so that the inside is towards your face and the snorkel is on the left side.
2. Using your thumbs, position the strap above the skirt.
3. Place the mask against your face and clear the mask. Clearing the mask before you pull the strap over your head helps prevent water from being forced up into your nose.
4. Pull the strap up over your head, and clear the mask again if necessary.