It's a long story. But you need a pinion puller before you can do anything. Find one from a hobby shop that sells slot cars.
Once you have one, take the pinion off, desolder the caps from the motor can, and bend the metal tabs out to open it. Remove the armature, desolder the wire on it, and replace with slightly fewer turns of slightly thicker wire. Don't go overboard. Re-assemble the motor, solder the caps back on, and press on the pinion.
Easy, huh
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