This is for the Porter Cable PC305TP, but I'm sure it's pretty much just as easy for similar style planers. 

I started by removing the guard on the outfeed side of the planer. There are two thumb screws (arrows) that get removed in order to remove the guard. 

In order to remove the blades I used a T25 bit. There is one located here (image) on the planer if you don't have one. 

There are 6 screws to remove in order to remove the metal bar that holds down each blade. Note, when removing this bar, there is a spring underneath on each end.

Then I carefully removed the old blade and replaced it with a new one. 

With the new blade installed, I put the planer back together in reverse order of the previous steps.