Find a suitable position in the box, use the appropriate tools to open a 13.5 x 5.5mm rectangular hole, and place the USB plug with the pins facing up to make it easier to solder the wires. The socket may need to be secured with hot melt glue. and finally pins 1 and 4 of the new socket are soldered to the existing 5v+- output with wiring.
Are there rechargeable batteries in the box?