I have been using this trick for years and it will normally extend the life of the planer knives three fold. Nothing fancy here, not much that can go wrong.
-Sharpening stones. I use a diamond set that has served me well.
-Please unplug the tool.
-Even old blades can be sharp enough to cut you, be careful not to brush up against them.
-When you remove material from the back side of the knives the blades will get narrower. Do not remove so much that the blade clamps, bolts or cutter head strike the wood. Cutting will get crazy loud if this happens and it will be very hard on the equipment.
-Make sure you properly secure the blades. Follow the instructions provided with the tool to make sure the blades are properly clamped in place. I recommend double checking everything after.
This is not the same as a set of new blades but can cut just as well if properly sharpened. Do not try to grind out nicks or chips, there isn't enough material, simply offset the nicks or flip one of the blades over. Be careful with all of the projects you undertake and do not try something you are not sure you can accomplish.