Raise the metal wiper arm away from the windshield. It should be able to hold steady in a position perpendicular to the windshield. Be careful with the positioning; the metal wiper arm is spring-loaded, and it could snap back and crack your windshield.
Unhook the old wiper blade. Look at the joint where the rubber wiper blade meets the metal arm. There should be a small plastic stopper holding the blade in place. Press the stopper and unhook the old wiper blade to separate it from the metal arm.
Some wiper blades have pins to hold the rubber wiper blade in place, rather than a hook.
Make sure one hand is holding the wiper away from the windshield during this entire process.
You may want to protect your windshield with a folded towel, just in case you the arm snaps back while you're trying to change the wiper.
Insert the new wiper. Slide the refill wiper into the same end of the arm where you pulled the old wiper out. Gently pivot the new wiper until the hook snaps into place to secure it. Lay the wiper back against the windshield.