Does the change in notice period policy after agreement with employer is applicable on the employee without signature ?

My company has done a duly signed contract or agreement with me on the date of my joining stating that I have to serve for 60 days notice period from the date of my resignation submitted to the employer. The employer has made changes in the notice period policy that the notice period will be of 90 days now onwards after my agreement and mailed the changed policy document to all the employees. The employer has not made any revised agreement with me stating this new notice period policy change and also have not given or taken any sign on any revised agreement. After 1.5 year completion with the employer i resigned and I have also served for 60 days as said in my signed contract with the employer. Now the employer gave me a legal notice to settle with the full and final settlement which points out that I have to pay 1 month gross salary , and 15% intrest of total to be paid amount to the company against not serving for 90 days. Should I have to panic with fear of legal matters and pay the amount? How to write back to the employer's legal notice with refrence to legal laws?