Before you touch anything
Photograph the module, the cabinet label, and the cable positions. Note which cable goes to which connector. The connectors are usually keyed, but the routing order matters when you put it back.
Step one: power down that cabinet, not the wall
Most controllers allow a single cabinet to be powered off while the rest runs. If yours does not, power down the wall but leave the processor on so you keep the mapping.
Step two: release the module
Most indoor modules are held by four magnetic or screw fasteners. Use the suction tool, not a flat screwdriver. A screwdriver will chip the mask within the first attempt and that chip is permanent.
Step three: disconnect in order
Signal first, then power. Reconnect in reverse: power, then signal. Doing it the other way can put a surge through the receiving card.
Step four: check the replacement before fitting
Put the new module face down on a clean surface and compare the connector orientation with the old one. Modules from a different batch can have the connector on the opposite side. Fitting it anyway will damage the pins.
Step five: fit and seat
Seat the module from one corner, then the opposite corner, then the middle. Do not press on the LED face. Press on the frame edge only.
Step six: power and test
Power the cabinet, display full red, full green, full blue, then white. A module with a swapped ribbon shows one colour missing. That is almost always the ribbon, not the module.
Step seven: brightness match
New modules are brighter than modules that have run for two years. If the controller supports per-module gain, reduce the new module to match. If it does not, expect a visible bright square for a few months until the new one ages.
The mistake to avoid
Do not leave the old module loose inside the cabinet. It will slide, touch a power supply, and cause a short. Label it, bag it, and take it out.