Just purchased a Stanley fatMax Xtreme Model 10-789 dual blade utility knife. I can't fiqure out how to insert new blades into the knife.

I have the same utility blade with yours.

It only goes in one way, try to loosen the screw a little

How do I change the blade on the quick change/instant change knife? Warning: Blades are sharp and can cause serious injury. Be extremely careful when handling blades. Wear eye protection and dispose of used blades properly, such as wrapping in duct tape.


FULLY EXTEND THE BLADE.
Press down and forward again on the slide button until the button is in the full front of the top slot (0 through 3 in the below picture).

Note- the blade must be extended beyond the 3rd detent position. This will cause the slide button to remain in a down position, allowing the blade release button to operate.

PRESS AND HOLD THE BLADE RELEASE BUTTON ON THE FRONT SIDE OF THE KNIFE. (Step 4 in the below picture)

Note- if the blade is not fully extended beyond position 3) , the blade release button will not work.

Remove the old blade carefully.
Insert new blade.

NOW, RELEASE THE BLADE RELEASE BUTTON.
ENSURE THE BLADE IS LOCKED IN PLACE. If the blade does not retract, try carefully giving a slight tug on the blade to reseat.

On the side there is a big gold wheel, twist counter clockwise and the case should split in half, the blade is held by a magnet, prise the old blade off and replace with new one, then put case back together and tighten gold wheel clockwise
Goodluck!

The model in your picture is a fixed blade utility knife (model 10-816). Your question asks for instructions for model 10-778, which is a retractable knife. I'll guide you through the 10-778 which you asked for, if you need instructions for the pictured 10-816 please re-post the question.

Push the thumb slide forward as far as it goes exposing the blade. Press the blade change button on the side of the knife (the small button on the left side near the blade) and pull out the old blade.
Insert the new blade and slide thumb slide back to fully engage blade in the internal lock.

some stanley knifes require stanley blades only, or make sure the clip blade is in is seated in the grove completely.

open the blade all the way and turn the knob to loosen, then scissor the two halves to open

Please watch tthis video.
https://www.youtube.com/watch?v=TsKu2DGVu50

1. Make sure the side lock is unlocked.
2. Push the thumb tab all the way to the front of the blade handle.
3. Insert the blade in the opening, you may notice two notches on the top side of the blade.
4. Press the small blade release button as you slide the blade into the knife. You may have to jiggle up and down to get the two notches on the blade fully into the handle.

If properly inserted, the exposed area of the blade should look very close to an equilateral triangle.

make sure both notches of the blade are inside the handle. They notch into the thumb lever.

good luck

Are you using Stanley Blades or off brand? Hey, I use off brand but sometimes it does make a difference. Second, make sure the blade slider is clean. If you were cutting shingles, for instance, the tar gets in there and screws things up.. try WD-40 spray it liberally all over the blade slide and buttons. If that doesn't work and you purchased it at a Menards store, I can speak of experience they'll replace it no hassle.
I hope I helped