Fraud on Agreement Paper of Personal Loan of ICICI Bank

I took a top up on my Personal Loan on ICICI Bank, with intention to reduce Interest Rate and Tenure, but I signed almost a blank agreement with the Principal, Interest and Tenure filled in. The bank increased the tenure by overwriting without my countersignature in that place, and disbursed the loan. I made several complaints to their grievance cell, and also Banking OMBUDSMAN, but they are giving the same reason that no. of installments, and start and end date of EMI are all correct and the overwriting was done after taking my consent. I requested to close the loan, agreeing to pay the entire principal but no other charges. Bank not agreeing. I have not paid any EMI till now., and bank now sent a advocate's letter threatening civil and/or criminal case against me. What steps can I take? What are my chances against the bank if I go for a legal tussle? I have the details, with correct tenure, written to me and signed in a blank paper by the sales agent before taking the loan.