Use the point of a spudger to push on alternating sides of the connector to "walk" it out of its socket and disconnect the battery from the motherboard.

It may take a lot of alternating before the connector comes out. Be patient.

If that doesn't work, grip all the cables at once as close to the connector as possible and pull the connector slowly and steadily out of its socket.

Lift the battery from the bottom edge to to remove it from the Chromebook.