If you have the UK plug power supply:
First remove the original connector:
With the terminals closest to the bench, tin the 2 outer terminals of the USB plug:
Strip the outer insulation of the PSU cable back about 20mm then strip and tin the wire ends. Solder them as shown to the USB plug.
NOTE if your supply has GREEN and BLACK wires, connect the GREEN wire in place of RED and BLACK in place of GREEN.
Finally use the 2 black plastic housing pieces to finish the plug assembly.