Without hearing you speak it's difficult to say what you should work on. Indian English embraces native speakers of many different languages and dialects, and each brings different problems to English pronunciation.
With respect to phonology—pronunciation of individual sounds, what you call ‘alphabets’†—this Wikipedia article may help you identify your own points of difficulty.
But in general I‘m going to guess that the biggest problem your hearers face is not your pronunciation of individual sounds but the tonal contour of your phrases and sentences—what linguists call ‘prosody’ or (as in the linked article) ‘supra-segmentals’. English listeners tolerate a great deal of variety in the pronunciation of phonemes, but rely very heavily on stress patterns to identify the ‘shape’ of sentences; and as the article tells you, Indian languages use stress very differently.
To attack this problem I suggest simple imitation. Find recordings of fairly long passages by native speakers of the particular dialect you wish to emulate—General American or Australian or British Received Pronunciation or Estuary English, or whatever. The recordings should be fairly conversational in tone, not readings from technical or highly ‘literary’ works; interviews with practised public speakers will do very well, particularly if they are telling stories rather than just giving brief answers to questions. Sit down with the recordings, for twenty or thirty minutes at a time, playing stretches of two or three sentences or so, and try to reproduce exactly what you hear. It will feel very odd and artificial for quite a while, but at some point everything will ‘click’: your voice and the recording will have the same lilt and feel. You will then find it very natural to carry that lilt and feel over into your own speech.
That, at any rate, is how I used to learn dialects for stage use. And you should think of it that way, as a role you are playing. You are 'portraying' an English speaker: not losing an Indian accent, but acquiring a specific English accent.
† This is a problem of a different sort, a lexical one. Alphabet is a common ‘Indianism’ for Standard English letter. And since English spelling (as you are no doubt painfully aware!) is very far from being phonetic, letter is really not appropriate when speaking of pronunciation.