You will need to desolder the two contact points that connect the battery to the mother board.

Make sure to allow ample time (at least 5-8 minutes or more) for the soldering iron to heat up. Otherwise, the solder won't melt.

Touch the hot iron to the contact points and gently pull on the wires to loosen them from the solder. They should release easily.

Make sure not to touch the contact points after desoldering them because they will remain very hot for a short while afterwards.