The original planer blades are set up and leveled at the factory. However, when you've re-sharpened High Speed Steel (HSS) blades and are reinserting them you will need to set up them again. You will need to buy a leveling device. Worn or dull HSS blades can cheaply be re-sharpened, so it's worth it. Full blade replacement is only necessary when a lot of material has been removed from the tips of the blades. Blades are installed in pairs and they should be replaced together to avoid excessive vibration.
Place both the blade and blade retainer on the leveling fixture. Check to see that the blade retainer engages in the grooves.
Press the plane blade against the stop in front of the cutting edge.
Tighten the blade retainer onto the blade with the locking screws. This adjusts the blade to the correct height and level.
Tighten the retainer screws.
Remove the blade and retainer all assembled from the leveling fixture.
Repeat with the second blade. The blades are now set-up for reinstalling into the planer.