Retractable utility knives come from a variety of manufacturers and in many shapes in sizes. Most have spare blades secured inside the handle. You can also purchase extra blades from a number of sources. Be sure to purchase the same type of blade that came with your retractable utility knife, even if the replacement was made by a different manufacturer. The replacement blades must have the same layout as the originals to work properly.
Changing the Blade
Retract the blade into the handle. Unscrew or open the handle to expose the used blade. This is best accomplished on a flat surface, such as a table. Lift the used blade out of position. It can still be sharp enough to cut you, so use caution when handling it. Set the used blade aside. Take a new blade from either the package or the handle and insert it into the proper position. Be sure that it is set properly on any sliding elements to which the old blade was attached. If there is not a good place to dispose of the used blade, place it in the handle with the extra sharp blades. This will keep it secure until you can dispose of it properly. Re-secure the handle by either tightening down the screw or closing it per the manufacturer's instructions. Your retractable utility knife is ready to go back to work.