Plug in Soldering iron and wait for it to heat up.If your soldering iron has a dial, adjust the setting somewhere between 15 and 40 Watts .

Next position prepped battery wires to the same spot where the previous wires were removed from.

Then Touch the soldering iron to the twisted copper wire for about a second to heat the wire.

Next, touch the solder to the soldering iron, so that a small amount of solder melts and forms a small bead between the surfaces being connected. (Avoid pooling of solder material by keeping this step between 3-4 seconds)

Repeat this step for the second wire. Secure new battery and place it in the position the original battery was in. Secure using the device tape, and screws.