There are a couple ways to replace a garage door roller. Most important, Do Not Ever take the bottom fixture off. The bottom fixture has a cable attached to it that is under very high tension.
If your garage door roller needs to be replaced you can take the hinge off and remove the old roller then install the new roller and put the hinge back on. This method is an easy way to replace the roller but more time consuming and you will need an impact or screw gun.
If you do not want to remove the hinge then you will need a pair of vise grips and a hammer. Determine the roller that needs to be changed and right above it bend the track out with the vise grips. Bend the track just enough so when you lift the garage door the roller starts to come out. Now put the claw of the hammer behind the roller and pop it out toward you. Remove the old roller and install the new roller. Now put the roller back in the track and bend the track back straight with the vise grips our use the hammer.