I don't have a manual for the tomahawk chipper and I cannot figure out how to change the blades.

You will need to turn the flails (also called cylinder hammers).  Each flail has  4 sharp corner cutouts, only one of which is in use.  When you turn them you get to use a fresh corner, the dull one is still there but no longer contacts the stuff you are chipping.  You will need to remove the chute to access the flywheel, and by rotating the flywheel access the steel bars (cylinder pins) that hold the flails and spacers.  I lay the chipper down, with a 5 gallon bucket protecting the engine, gas tank, to access the bottom, so that you can, using a punch, drive out the 3/16X3/4 roll pin which can be found in one of the long spacers.  You will note each bar has 4 flails, with two long spacers and two short spacers and the roll pin goes through one of the long spacers.  You buy the roll pin punch at a hardware store.   There are four sets of bars with spacers and flails, so four roll pins.  You will want four fresh roll pins also, because the old ones are compressed and shouldn't be used again.  When you have punched out the roll pin you can then drive the bar from the end (where the chute was) but be sure to line the other end of the bar up with the hole on the other sidewall of the chipping compartment.   When you are trying to punch out the roll pin it helps to stabilize the fly wheel--I use a couple of vise grips in a way that the fly wheel is stable.  When you are pounding out the bar (called a cylinder pin) you will need a slightly lesser diameter long (6-8") bolt as a driver.  You can pound on it and it will follow the cylinder pin as it gets pushed out the other side.   When the spacers and flails fall out onto the floor, then you can turn the flails to use a fresh cutting corner and put the spacers and flails back on.  I find it helpful to file the rough edges of the inside ends of the spacers so they slide easily on the cylinder pins.  Get a round file for this purpose.  Also, keep some WD 40 on hand because these things tend to be stuck.  I do only one cylinder pin (with its four flails and four spacers) at a time, so I correctly put it all back in the right order.   You will see that the the order alternates.)