How to Restore Leather

My question is not exactly about car seats. But hey, I figure there has to be someone on here who has restored leather seats for his car, and will know the answer.

I have a white leather sofa that has turned a dingy yellow and has several stains on it. No one, and I mean NO ONE where I live knows how to clean stuff like this. There would also appear to be no high quality products available.

Perhaps someone here has restored leather seats on their Porsche and might know how I can clean and try to rejuvenate my sofa using stuff I can buy at the local store? Thanks