To change the tube, first you have to take the wheel off the bracket. To do this remove the entire front wheel assembly, whether by removing the two screws on the under side or by unlatching it, if you have the newer model. Remove the two Phillips head screws on both sides of the bracket that hold the wheel. The wheel will slide out of the bracket. It might require two people to do this, one holding the wheel and one holding the long part of the assembly and pulling the wheel off of the bracket. Once the wheel is off the bracket, you can change the tube.