Unplug the power hacksaw from its power source. Make sure the saw is unplugged and turned off before attempting to change the blade as touching the blade with the power on could result in serious injury.
Locate the wing nuts on the front and rear ends of the hacksaw blade assembly. Loosen the nuts by hand until the blade is loose enough for you to slide it out of the blade frame.
Insert the new blade into the blade frame so that the blade teeth face away from the blade handle. Hook the holes on the front and back of the blade onto the small pins located at the front and rear of the blade frame. Tighten the wing nuts to hold the blade securely in place on the blade frame.
Plug the power hacksaw in and turn it on. Wear appropriate personal safety equipment (safety goggles and gloves) and make a practice cut onto scrap metal to test the tension of the blade. Watch the blade during the cut to see if it arches or bows; turn the machine off, unplug it and loosen the wing nuts slightly to correct the tension.