I do not know how to change the blade on a dewalt
I'm not familiar with that particular band saw but I have had a few different band saws and they all pretty much work the same.
Unplug the saw and remove the top and bottom wheel guards.
Look behind the top wheel and you should see a large bolt standing vertically it may have a knob on it or it may not, look on the bolt to see if it has stop nuts if so loosen them then turn this knob so as to lower the wheel enough so you can remove the old blade.
Clean the rubber tires on both wheels and install the band with the teeth facing down.
Now the tricky part, adjust the top wheel so that it is firmly against the band and turn the wheel slowly by hand (be sure the saw is unplugged) if the band "walks" one way or another, you will have to find the set screw or set screws that adjust the tilt of the wheel, adjust these until the band runs true on the wheel. When it does, tighten the wheel a litte more and turn again slowly by hand repeat this until the band runs true and it is very tight on the wheel, Put the guards back on before you turn on the power. You should be good to go. Read More At : dw3988 14tpi portable band saw blade - 44-7/8", .025" - 3......