You most certainly can replace just the refills. It is pretty simple actually. First things first, make sure you have a Toyota blade assembly. If the truck was bought used it might have Anco blades or some other off brand. If that is the case, you would be better off just getting the entire blade from Toyota.
Insert replacement
On one end, there may be a silver end piece about the size of an eraser. If present, remove with needlenose pliers.
On the opposite end, there is a thicker part to the rubber insert. Pull from that end, and the entire insert should come out. Remove and keep the metal stiffeners from either side.
Put those stiffeners on the new insert, and slide it in the blade backing. The narrow end will go in first. When the meaty end gets to the edge of the blade backing, just give a gentle tug to lock it into place. Put the new silver end piece on the narrow end and you are good to go. Repeat for other side.
For entire blade assembly
Pull arm up, go to the end and you will feel a small clip inside the "C" of the arm itself. Press upwards towards the arm and pull the wiper blade down at the same time.
Installation is the reverse of removal. Just make sure you hear the click so they don't fly over your roof in a rain storm going down the interstate.
