When a motor spins backward after a crash, it usually means the wiring connection between the motor and ESC has changed somehow. During impact, wires can get yanked loose and reconnected incorrectly, or a solder joint might break and be hastily repaired in the wrong sequence. The good news is you can fix this mechanically in about thirty seconds without touching any software.
Brushless motors spin based on the sequence of electrical phases fed through their three wires. By swapping any two of these wires, you reverse the phase sequence and flip the motor's rotation direction. It doesn't matter which two wires you swap. If your motor has blue, yellow, and black wires going to the ESC, you could swap blue with yellow, blue with black, or yellow with black. Any combination works equally well.
First, identify which motor is spinning wrong. Power up your quad with props off and arm it gently to verify. Once confirmed, disconnect the battery. Locate where the three motor wires connect to the ESC. Most builds use bullet connectors or direct soldering. If you have bullet connectors, simply unplug two wires and swap their positions. Push them back firmly until you hear a click. If they're soldered, you'll need to desolder two wires, cross them, and resolder them to the opposite pads.
After swapping, do a quick continuity check if possible. Make sure your solder joints are solid and wires aren't touching each other or the frame. Reconnect the battery, arm the quad without props, and test again. The motor should now spin in the correct direction matching your other motors.
Some pilots worry about mixing up motor numbers or ESC assignments, but remember that your flight controller doesn't care which physical wire goes where. It only cares that when it sends a signal to ESC number two, for example, that motor spins clockwise or counterclockwise as configured in your software. The wire swap is purely a hardware correction that aligns the physical reality with what your flight controller expects.
This method beats reflashing because it's faster, doesn't require a computer or configurator software, and can be done trackside at a race. I've fixed reversed motors in the pits between heats dozens of times this way. Keep a small soldering iron in your field kit and you can handle this repair anywhere.
