Position a step ladder under one of the garage door top fixtures. The top fixture is the bracket that holds the stem of the top roller. If you have a standard door, the fixture looks like a tent. Low-headroom doors have a flat top fixture.
Loosen the nuts securing the slide to the top of the fixture with a 7/16-inch socket wrench. The roller stem slides into the sleeve of the slide. The slide moves in and out for adjusting the top section. Only loosen the nuts one full turn.
Press the top section of the door against the wood stop of the opening frame. Move the slide as necessary to hold the section against the frame. Tighten the nuts to secure the slide to the fixture. Do the same thing to the top fixture on the opposite side.
Loosen the nuts or bolts securing the flat top fixture if you have a low-headroom door with a double horizontal track. Push the flat bracket down slightly. This pushes the door section against the opening stop. Tighten the bolts or nuts. Do the same thing to the top fixture on the opposite side.