How do I disconnect (and reconnect) the tubing from this fitting?
Just to add some visuals to the others' explanations using your photo, you need to (1) push on the collar trying to move it parallel to the tube, and (2) pull the tube.

To reconnect, push the tube into the collar until it stops (should be about 1 inch). Make sure the tube end is square (perpendicular to the tube axis, with no damage); cut a bit of the tube if necessary.