These are an advanced case of spider mites. The normal colour of the leaves will never return. For a quick and effective solution:
- make sure the plant is located where there is lots of light. A south east window is ideal.
- remove every leaf cleanly from the base of the stem where it meets the trunk
- get a cloth or paper towel soaked in a little bit of dish soap and water and run it over all parts of the plant that are left
- repeat the application of soap and water every five to seven days at least twice or three times
- wait for new growth, inspect the underside of the leaves for spider mites that look a little bit like grains of salt