Ryobi power planer blades are reversible; you get two blades for the price of one. When the blade becomes dulled, switch it over and use the other side. Blades for the Ryobi planers come in pairs; always switch them out or reverse them in pairs. Unlike old hand-planer blades, you can't sharpen Ryobi blades, so replace them when they appear damaged or chipped. Ryobi manufactures hand planers and stationary thickness planers; the procedure for removing blades on them are different.
Unplug the planer—a basic safety rule for hand tools, particularly one with a blade.
Turn the planer upside down and secure it as well as possible. If you have a wooden vise, put the tool in that and tighten the vise enough just enough to hold it. Don't put the tool in a metal vise -- you can crack the housing.
Loosen the three screws that secure the blade. (Your planer should have come with a wrench to do this; check the box.) Just loosen the screws—don't remove them.
Look at the direction of cut and make a mental note. Then observe and replicate the tapering of the blades. Place the tapered edge of the new blade on the same side as the screw heads; the flat edge faces the cutter block.
Push the blade out of the blade holder with a flat-head screwdriver—it should go to the right.
Remove the old blade. If necessary, tap the blade with a block of wood to loosen it.
Slide the new blade, then orient and center it using the screwdriver. Retighten the three screws.
Repeat with the second blade.
Unplug the planer. Remove the chip guard from the infeed side of the machine by unscrew the two wing nuts securing it.
Loosen the lock bar holding the blade in place by turning the two lock bar screws on the side of the machine counterclockwise, using a screwdriver. The blade is spring-loaded and pops out when you do this. Pull it out of the planer, then pull out the lock bar.
Set the lock bar back in place against the cutter head and position the blade against it.
Tighten the lock screws on the side of the machine to hold the blade in place.
Reinstall the chip guard and tighten the wings screw holding it.