Suntan is caused by exposure to sunlight without protection. The UVA and UVB rays of sun (partly visible light too) enter the skin and stimulate formation and distribution of excess melanin which makes the skin darker. Continuous sunexposure can worsen tan and even cause sunburns, rarely skin cancers.

If the skin is protected well after accidental exposure, the tan gradually fades over weeks to months even without treatment.

However, removal of tan is a very common problem that a lot of people face. They usually resort to OTC products, natural remedies like lemon juice and visiting parlors for facials. While lemon juice, turmeric etc do have skin lightening properties, they need to be used correctly and for a long time for good results while OTC products often have an inconsistent result on different people.
Facials cannot remove tan in single sessions. Good quality facials can exfoliate the skin and cause temporary shine, but they cannot penetrate to the layer inside and act on melanin. Often the parlors use harsh chemicals that can cause irritation and burns and even worsen the skin quality, specially on an already damaged skin.

The best way to treat tan is to prevent tanning, i.e. using sunprotection by physical means like umbrella, along with use of medical grade sunscreens with a broad spectrum action. In case you have an existing tan, use a good skincare regimen with sunscreen, mild cleanser, moisturiser and visit a dermatologist for treatment. Skin lightening creams, chemical peels and even lasers can be used judiciously for great and fast results in tan removal.