If the plant in your tank turns yellow, it's most likely that the plant has died. If you recently purchased your BioSphere, please reach out to the store that you purchased it from and inquire about their warranty policy.
For the safety of the BioSphere, you want to remove the plant. The plant can be easily removed from the tank by gently pulling it straight up.
You can replace your plant with any 4-inch Lucky Bamboo, Java Fern, Sagittaria Grass, Hornwort, or Anubias - that you can typically find at a pet store that sells aquatic animals. To add Lucky Bamboo or Sagitarra Grass, push the plant down into the gravel/sand with your fingers, tucking the roots into the sand. Once it has hit the bottom of the tank, gently push the decorative rocks around the plant to hold it in place - while the roots grow into the colored gravel and sand. It's okay if some of the roots are exposed.