Citizen manufactures a large range of watches both for dress and for sports such as scuba diving. They are high quality and use high-quality parts. Although the crystal used in the face is scratch-resistant, if it does become damaged it will need to be removed and replaced. This will require knowing the watch type and serial number to obtain a crystal that fits correctly.
Locate the serial number of your Citizen watch. This is found on the back of the watch below the Citizen logo. Citizen watches need to be replaced with crystals supplied solely by Citizen. Contact a jeweler or an online distributor, and provide them with the serial number to obtain the crystal.
Remove the watch back. Depending on the style of watch you will need to unscrew, pry or remove screws from the back of the watch.
Remove the old crystal. Depending on the style of watch, you might need to use a crystal lift, which you tighten around the edge of the crystal and lift it out. If the connection is loose then place a cloth over your hand and simply push out the face. If the face is glued in, you will need to use a chemical component such as "Attack" to dissolve the glue and then press the face out. For completely metal designs, you can use a heat gun to melt the glue and remove the crystal.
Replace the crystal gasket which seals the watch. Cover the gasket in a light oil or silicone grease to consolidate the seal.
Apply the new crystal and seal it in place with a watch crystal press or jeweler's crystal epoxy cement.