The connector for the RTC battery is similar to the speaker and fan connectors. Before removing it, lift the silver flap that's also stuck to a wide cable connected to the motherboard.

To unplug the battery, pull on the connector in the direction of the arrows. It is a similar connector to the speaker and fan connectors. In my case, it required a bit more effort to get it out.

Finally lift and unstick the battery from it's holder and put it aside.