The soldering iron can destroy components on the motherboard. Only touch the soldering iron to the solder already on the board.

Touch the tip of the soldering iron to the solder connecting the positive (red) wire.

Pull back on the wire with the iron in place, removing it from the hot solder.

Repeat this step for the negative wire.

The solder will be melted almost instantly. There is no need to hold the iron on the solder for than a second or two. Visit IFixit's soldering guide for more info.