Replacing the trolley rack on a screw drive garage door opener is fairly simple. Instructions do not come with the part. I am sending you instructions based on my experience. If you have farther questions send another email.
Turn the down travel adjustment 3 full turns toward the down arrow. This will allow the trolley and rack to run off the end of the track. You need to stop the garage door opener when the rack clears the end of the track.
Pull the release rope on the trolley and put the door all the way down.
Remove the pin that holds the rail to the bracket on the wall above the door. There is a clip holding the pin in the bracket. This will need to be reinstalled when you are through.
Support the end of the rack on a ladder or something that will hold it just below the header that goes across the top of the door opening.
Run the garage door opener in the down direction until the trolley and rack come off the end of the rail.
Slide the new rack and the trolley on the end of the rail until the teeth on the rack engage the screw drive. Run the garage door opener in the up position until it is well onto the rail or until it stops at the up position.
Raise the rail and install the pin and clip and return the down travel adjustment 3 turn toward the up arrow.
Attach the door to the trolley and test the down and up travel. If the down travel is set too far down the door will close and reopen. You might have to tweak the down travel a little up or down.
I am sending you an image from the owners manual for installing the pin and ring that holds the rail to the wall bracket.