Make sure battery is safely secured and re-solder the battery contact tab onto new battery.

Check that contact tab is re-soldered onto the original solder.

Once solder is dried and cool, toggle the power switch (not the power button) to check that the device is functional.

Make sure solder adequately covers the contact tab so the solder will not break in the future

Solder with caution. Use gloves and eye protection. Solder contains lead, work in a properly ventilated area.