There will be an oval rubber plug at the bottom of the backing plate.
You can pop that plug out to rotate the ratcheting star wheel.
Sometimes the threads of the adjuster get all gummed up with brake dust and it will no longer work as it should.

When replacing rear shoes sometimes the drum won't come off because they have worn and there is now a lip inside the drum that gets caught on the shoes.
You'll need to back off the adjuster for clearance.
Turn the drum on a brake lathe to remove this lip before installing new shoes. That way, you can roughly adjust before putting the drum on.

With the drums back on, apply the brake pedal a few times to center the shoes inside the drum. -Then- adjust until the drum starts to drag while you turn it by hand.

Tools are available that offer more clearance than a screwdriver.
They look like a double ended flat bladed screwdriver and are bent to clear the axle so you can lever on the adjuster opening.
We always called them Brake Spoons.
The one I have in my box that works best for my truck is a Snap-on B3404B.



Hope this helps....