If anybody ask me "How are you?" then how should I reply?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The Sunnah for greeting is to say: "As Salaamu Alaikum". The complete greeting is "As Salaamu Alaikum Wa Rahmat ul-lahi Wa Barakatuhu". It is reported from the Prophet that he did not allow the one who did not greet with Salaam into his presence. Salaam means being free from all kinds of defects. Arabs in pre-Islamic time used to greet by many words such as "Blessed your morning" "Be away from curse" and the like expressions. But Islam teaches us the best way for greeting. So, if one greets you by those words, which are not proved from Sunnah, then you have to make him aware of Sunnah. Bring to his notice that Sunnah is to pronounce Salaam before beginning any conversation.
Allah knows best.