Suntan is definitely reversible and to understand how you need to know the mechanism behind the issue. When UVA and UVB lights in sunrays enter the skin, they trigger production and distribution of excess melanin pigment which makes the skin darker. So the reversal of suntan would involve preventing this excess melanin formation and removing the deposited melanin.
If strict sunprotection is maintained every time you go out, the skin can recover itself but it takes weeks to months. Such protection would involve sun barriers like umbrellas, covered clothes, hata, avoiding sunlight at peak hours, and use of sunscreens. Sunscreens with both UVA (PA+++ and above) and UVB (SPF30 and above) are needed and it should be reapplied every few hours for optimum protection.

For removing existing tan, doctor may prescribe you skin lightening products containing glycolic acid, kojic acid, arbutin, liquorice extract etc for regular use. These products can remove the melanin from top skin layers and prevent formation of new pigment. Chemical peels are another modality where exfoliating products like high concentration glycolic acid, kojic acid, trichloroacetic acid etc can be used under medical supervision to remove top damaged skin layer and brighten the skin from below. For deep pigments, Q switched Nd:YAG lasers can help break down melanin when other treatments have no effect. Please visit a dermatologist for your concern. He/she can check your skin, decide on a skincare regimen and proper treatment plan which can give you fast and satisfactory results for your tan removal.