This is a very simple job to do yourself in fact all you need to do is follow this simple 6 step guide.
Tighten garage door belt.
Brand new chamberlain garage door opener with excessive slack in belt drive.
The belt needs to be connected with the tensioner loosened so that the tensioner can be used to take up any excess slack.
All you need to get started is a step ladder a 7 16 inch wrench and a flat head screwdriver.
1 close the garage door.
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.
Step 1 closing the garage door before beginning use the garage door opener to close the door and fasten it.
The specific procedure should be in the manual for your unit.
Loosen the adjuster three turns counterclockwise if the door raised without.