Here are the instructions (abbreviated) for removing and installing backing plates:
1-Safely lift trailer on side to be worked on, and block wheels not being worked on.
2-Remove hubcap, remove lug nuts, remove wheel/tire assembly.
3-Remove dust cap from center of hub/brake drum assembly.
4-Remove cotter pin holding castle nut in place, and unscrew castle nut.
5-Wiggle drum slightly to loosen outer bearing, and pull bearing out of hub.
6-Slide drum off spindle, remove inner grease seal.
7-Uncap the two wires for the brake magnet, and separate the wires.
8-Unbolt the backing plate from the spindle, there should be either 4 or 5 bolts with nuts holding the backing plates to the spindle.
9-Slide backing plate off spindle.
10-Slide new backing plate onto spindle, making sure backing plate is for correct side of trailer, the arm holding the magnet should go toward the front of the trailer.
11-Reinstall the mounting bolts, and connect the wires back together using new wire nuts.
12-Repack bearings after inspecting them for damage, install the inner bearing, then install a new grease seal.
13-Slide brake drum assembly onto spindle, and slide outer bearing onto spindle.
14-Reinstall washer and castle nut, tightening to specification, then install a new cotter pin.
15-Reinstall dust cap, and tire/wheel assembly.
16-Adjust brakes to specification (usually a slight scrape when rotating the wheel).
17-Lower trailer, and torque lug nuts to specification.
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