Most likely it has a 3-button stack or "cylinder battery", 3.6v 50mah.
They are easy to remove by rocking it back and forth until it breaks loose from the motherboard. Generally these do not have a plug for external battery, but there may be a few exceptions. The jumper near the battery is a cmos reset and not a battery connector.

You will need to solder to reattach the new battery, so if you havn't done that a lot, you might ask someone who has to do it for you.
Easiest replacement method is to us a "Button Coin Cell Battery Holder Case"(ebay) and a CR2032 button battery, as that has leads and is easy to solder.