Catching his breath alongside a road near Seattle, WA USA
1999 K2 OzM, 2001 Aegis Aro Svelte
Shimano brakes have a little lever off to the left side that acts as the brake release. Just flip it up (rotate clockwise) and the brake calipers will spread apart far enough to allow you to remove your wheel. Note that some tyres may be a little too wide to properly clear the brake pads even with the calipers fully open. In that case, you will need to play with the cable adjustment knob to release more cable. Ideally, your brakes should be set up so that with the release fully open you can remove the wheel with no problems and likewise when the release is closed, the pads should be positioned a scant few millimeters from touching the braking surface.