Grip, and gently pull outwards on the cable as close to the battery connector as you can

Pull in level with the back cover in the direction as indicated by the arrow (not upwards, and not sideways), otherwise you could damage the connector, the cable, or just rip everything off.

When plugging the battery back in, make sure to use a non-conductive tool with a flat head to push the battery plug back into the connector, to avoid shorting the battery pins in the plug.