Turn on the forklift and raise the mast up so it is 4 inches off the ground. Locate the bolts on the side of the mast. The mast is found on the front of the forklift. The forks come out of the mast and it is very heavy when you remove it from the forklift.
Remove the first bolt on either side of the mast with an adjustable wrench. Turn the wrench to the left to begin to loosen the bolt. Continue turning until the bolt becomes loose enough that you can turn it by hand. Leave it in the forklift so it can hold the mast up while you remove the other bolts. Repeat the process for the bolt on the other side.
Locate any other bolts holding the mast in place. The location of these bolts vary depending on the manufacturer of your forklift. Loosen these bolts and remove them from their current locations. Set them aside for future use. Have a person hold the mast on each end and remove the two remaining bolts that you loosened in the previous step.
Set the mast down carefully and pick up the new mast. Have your helpers hold it at each corner while you attach it to the forklift. Tighten down the same bolts that you removed into the exact same locations with an adjustable wrench. Continue until all the bolts have been installed.
Turn on the forklift. Raise and lower the mast to verify it is installed properly. Tilt the forks up and down. Pick up a load and verify it can support the weight. Turn off the forklift upon completion.