Turn off the water supply valve behind the toilet. Rotate the valve handle clockwise until it stops to turn off the water supply. Lift the toilet tank lid off the toilet and set it aside on a flat surface.
Grasp the top of the float with your fingers and hold it up slightly. This also lifts up the float arm that connects to the float and the Fluidmaster 400 cap.
Press down on the cap slightly with your free hand while holding the float. Turn the cap counterclockwise until it releases from the Fluidmaster. The cap will hang over the side of the fill valve once you remove it from the top.