So, you've tried to pump up a tire and found that it's flat, it's not holding pressure so you know you have to change the tube. The first step to take the wheel off the bike is to open the brake up. On a road bike with a side pole brake like this it will have a release mechanism to open the brakes up to allow the tire to pass by the pads. Other bikes will have brakes that look like this. This is called a linear pole brake. The step involved in opening this brake up is holding this little bracket at a right angle to this arm, hold it still, pull at this thing, called a noodle, to release it from it's bracket, then they'll open up. I'll do that again, so you can really get a good idea of how that mechanism works. That is it properly seated, a little boot covers that, extensively to keep dirt out. So again, to release this brake, push that over, hold this bracket at a right angle here, pull back on the noodle, it will come free of that slot, then the brake will open up. The third type of brake that you may come across of is this, this is a cantilever brake. This mechanism unhooks by grasping this yoke here firmly with one hand and pulling it out on the side of the brake here. That'll allow this type of brake to open up. Some bikes are equipped with the brake release located at the lever. In the case of this style of brake lever, it's right here. By pushing this pin inward it allows the lever to come up further thereby allowing the brake to open up far enough for you to remove the wheel.