You need to nuts that will screw onto the yoyo axle. These are nuts:
Once you have them, do all the steps:
1) Find two nuts that screw on tightly on the axle.
2) Screw the 1st nut about halfway through the axle. (Use pliers or something similar for the screwing so that you can grip the nut well)
3) Screw the 2nd nut until it reaches the 1st one, then make sure to really tighten it on top of the 1st nut.
4) Grip both nuts together at the same time, and move them as if you’re trying to screw them off. If you did the previous steps correctly it should start getting removed from the yoyo hub when you screw it apart.
5) To remove the nuts you now have, just grip onto the nut on top and screw it loose, and get them off one at a time.