how to convert english Excel file to Hindi in c# ?

English and Hindi are different languages. You cannot simply convert "A" to an equivalent Hindi character. You need to know what a word means and thus translation would act as the vital element for this project. 

There are some good libraries and services available online. You can try out any one of them. Google Translate? Bing Translate? Or you can also search on Google for any other third-party library.