Notice period query

I am working in a company for past three years(from 2017) and my appointment letter says one month as per notice period. After sometime(2019) i have received an email communication telling my notice period changed to two months and it doesn't came with any condition, its one way communication. I never accepted this via email and told i am not agreeing it. Now(2020) I wanted to resign but the HR people are telling my notice period as 2 months. My appointment letter as no such conditions. Can I fight this in labor court ? Will this work on my favor if i go to court ? Urgent please , i don't want to miss another offer. The email regarding change in notice period was never accepted by me in fact i responded that saying that i am not accepting and i will server one month as per my Appointment letter. Below is the eamil "In furtherance to the Appointment Letter issued, we would like to inform you that there is an amendment with reference to clause 5.1 that your notice period has been extended for 2 months instead 1 month as per the notification received from the client"