The push rod is just held in there by a rubber ring. There should be a new one with the master cylinder.
You just give it a pull and it will come out, that's assuming you have taken the boot off.
If you can't get it out for some reason, take the screw and retainer off the back of the master cylinder and pull the piston out. It will make a mess.... You have the push rod and piston in your hand now.... Put the piston (it's cast aluminum) in a vise and crack it by squeezing it. The pieces will fall off and you'll have the push rod.
You'll need to put the new rubber ring on and push it into the new master cylinder. I squirt it with a little silicone spray to make it easier.