Make sure the gun is unloaded.
Remove Grips,
Cock gun,
Capture spring at the end of the strut with a 1/2" piece of coat hanger.
Wiggle and sneak out the assembly, don't be afraid of giving it a little tug.
Place the assembly in a vice with the captured end up.
get your vice grips and grab the saddle and push it down a little to allow you to remove the capture pin.
Exchange the springs and reverse the above.
While you have the assembly apart I like to give the strut a good go over w/ file to 1000 grit, to smoooooth it up,,, and take a micro file to the saddle where the strut rides.
Before re-assembly I like to give the strut a wipe of tetra grease too.