After many charging cycles, your battery can wear out and eventually lose its ability to hold charge. When that happens, you will need to replace the battery to keep using your Mifi.
Remove the rear cover by pressing down on the cover and pulling it away from the device.
Place your finger on the back of the battery located near the edge of the device.
Push your finger into the battery and lift up to pop the battery out of the device.
Insert the new battery into the battery slot.
Make sure to place the battery in with the white arrows facing inward.
Re-install the rear cover by placing the inward end in it's slot.
Push down on the cover while sliding it into the slot to snap it back into place.