If its the same type as the Dexter drum brakes there should be a rubber plug that can be removed on the bottom inside of the fixed portion of the brake housing. Inside there will be a notched wheel that can be turned for adjusting the brakes. I can never remember which direction to go and usually pick the wrong one on the first try. There is a special built little wrench for this adjustment but a screwdriver works well.
Do a Google and you should be able to find lots of pictures which will illustrate.
Southwest Wheel Company - 8k Dexter Electric Brake
# 15 should be the adjuster.