The USB charging port is located on the bottom left of the motherboard. This is indicated with a red box in the first image.

If you need help with soldering refer to this soldering guide.

Place the end of the soldering iron onto one of the solder joints. Place the soldering wick onto the joint as it begins to liquify, the solder will run up the solder wick and off of the USB charging port.

Repeat this for each joint.