Question: how to replace bottom of garage door?
Answer: Measure your old garage door section. ... 
Measure your new garage door section. ... 
Unplug the power cord to the garage door opener. ... 
Disengage the opener from the door. ... 
Place the hook of a come along on the top of the track, as shown. ... 
Place the other hook of the come along on the outside hinge that is second from the bottom.