H1B Amendment process with no prior travel

I have a H1B visa which is valid until 31st March 2022. I’m still in India and was unable to travel since there was not opportunity with the sponsered client. I would like to understand until when do I have the time to apply for amendment.

H1B amendment should be done before the I94 expiry, while your H1B is still valid.

Does that mean I can file for amendment even before a day of expiry?

I would not do that late. Talk to your immigration lawyer and act per their advice.