Hold onto the globe with one hand and unscrew the set screws in the fixture so that they are loose enough to remove the globe using a screwdriver.
Lower the globe out of the fixture carefully to avoid damaging the globe, fixture or light bulb. Set the globe on a work surface.
Measure the diameter of the fitter on the lip of the globe where it fastens to the light fixture using a tape measure. Wrap the tape measure around the globe at its widest point to measure the circumference. Divide the circumference by 3.14 (Pi) to get the diameter. This is the diameter of the globe. Purchase a replacement globe based on these measurements.
Lift the replacement globe up and place it into the light fixture opening. Tighten the set screws with a screwdriver while holding the globe in place. The screws should be snug, but not so tight that they put pressure on the glass.