It may be necessary to change the battery if it's faulty or remove and insert it again if your router isn't responding.
Remove back cover
Turn the back of your router towards you. Take hold of the back cover and remove it.
Take hold of the bottom of the battery and remove it.
Place the battery in your router (top first), matching the battery contacts with the router contacts, and press the battery into place.
Replace back cover
Place the back cover on your router and press it into place.