Loosen the adjuster three turns counterclockwise if the door raised without.
Tighten garage door belt.
Step 1 closing the garage door before beginning use the garage door opener to close the door and fasten it.
The manual does not indicate how to solve this problem.
The specific procedure should be in the manual for your unit.
But basically assuming the belt is attached with enough slack removed the tensioner bolt should be able to taske up the rest of the slack once tightened.
The only thing you need to do with a belt driven opener is to adjust the belt occasionally.
All you need to get started is a step ladder a 7 16 inch wrench and a flat head screwdriver.
Tighten the adjuster three full turns clockwise to add tension if the door closed and did not remain open halfway.
This is a very simple job to do yourself in fact all you need to do is follow this simple 6 step guide.
The belt needs to be connected with the tensioner loosened so that the tensioner can be used to take up any excess slack.