Each of the 4 motors has 3 wires soldered to the motor control board. The colours should be the same as in the photo. Were you to accidentally reverse any two on reconnecting them, the corresponding motor would spin backwards, causing an instant crash on take-off.

Unsolder all 12 wires (or only the 6 on one side if you are simply replacing an arm).

Lead-free solder requires an extra hot iron to melt. It will melt much more easily if you add a little leaded solder. This will not only increase the thermal contact between the iron and the lead-free solder, but will also reduce its melting point as the solders mix. Use a solder sucker or solder wick to remove excess solder before reconnecting.

Lift off the connector at the top of the motor control board.

On reassembly, double check that this connector is fully engaged and pressed home (force is not required). If not fully engaged, you may find the drone appears completely dead. At the end of a couple of hours of intensive work on your precious toy this can cause an unpleasant sinking feeling in the pit of the stomach.