If you place a drift pin or other rod in the Vee of the spring and smack it downward with a small mallet, it will release the spring from the hook it is under. Do not have it cocked as shown using that method, you only cock it to collapse the spring if you use a clamp.
For that method, cock it, clamp it then decock it to remove the pressure so you can remove the triggerguard.
I tap the spring out, downward with the hammer down. Replace by squeezing the spring under the hammer and tapping it upward back under the hook, or replace the spring before installing the triggerguard and use the triggerguard screws to jack it back into tension. This is hard on the screw slots...