Now you should find you can raise your door using the opener without the chain jumping.
How to replace garage door opener sprocket.
Use a step ladder to access the garage door opener s power cord and motor unit.
Position a work table near the area under your motor unit to hold the garage door opener.
Fasten the sprocket cover back in place and plug in the power to the garage door opener.
Remove the chain and cable assembly.
This was a craftsman model 139 53976srt.
If it is just a gear you are replacing you should remove it after you have unbolted assembly and replace it with a new one by going in reverse order.
10 return to the floor and pull the manual release cord to refit the trolley to the chain arm putting your door opener back into automatic mode.
Lower the motor unit down.
This is a video on how to change the gear sprockets in a craftsman garage door opener.
If it is the entire assembly you need just replace the assembly with your new one.