Company changed notice period time by notification is it valid

I worked for a MNC company. As per my contract, notice period was 30 days, but in between company has changed the notice period for 90 days. Company has sent the notification through email. They have neither made any amendment in the contract nor taken any acceptance from me. I resigned from company & they asked me to serve notice period for 90 days but i told HR and reporting manager that my notice period is 30 days and i shall serve the same. But they did not accepted but escalation of matter to HR head they relived me on 45 th day.Now company is asking recovery against short notice period.