What are ways to fix holes in the teeth?

Depends. It really depends on the size and location of the holes in the teeth. Simple small shallow holes can be fixed by filling the tooth after cleaning of the decay(hole). Other deep holes may require root canal treatment to remove the infected nerve and fixe the tooth.
Fillings. If you have holes in your teeth then you have cavities. The only way to fix a cavity is to go to your dentist and get it filled.
Types and causes. Holes in the teeth are usually caused by bacteria. There are many types of ways to fill theses holes-porcelain, composite, silver-amalgam, and gold are often used.
Visit your dentist. Voids in teeth are usually caused by decay which weakens & demineralizes the enamel. Your dentist will remove the decay & restore the tooth with a filling.