Put the straw of the air canister under the grip.

Pinch the end (can be done with pliers), and use the gap created to slip the straw in.

To separate the grip, you may use a screwdriver or another long, skinny tool allowing you to gain leverage underneath the grip.

While spraying air underneath the grips, pull on the grips with enough force to move them off the handlebar.

The air will separate the grips from the handlebars.

If more force is needed, you may use an air compressor with a needle attachment.