No matter how tight the knot has been pulled, one needs only to "break its back:"

, The running end is the end with which you tie the knot (the ‘rabbit.) The standing end is the ‘tree’ around which the ‘rabbit’ runs. This spot where the ‘rabbit’ loops around the ‘tree’ forms the shape of a cross., You should push the loop away from the knot so that the back of the knot 'breaks'. This will release tension on the loop securing the running end and allow the knot to be untied.
 It may help to push the two sides of the knot together so that there is absolutely no tension on the rope while you pull the knot apart.
Undo a bowline knot.
Locate the spot where the "running" end loops around the “standing” part.
Push the loop up the standing part with your thumbs.
Pull the knot apart once the knot has loosened.
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