Nifty december options purchased 100, not able to buy back entire shares

I have shorted 100 units/4 lots of call option of strike price 8500 for the month of December. Now i couldnt buy back the 100 but only 75 as the lot size changed. what is my remedy for the -25 units. I recently entered into options trading. I am not aware the change in lot size. what are the consequences if i leave it for expiry.

You’ll have to leave it until expiry. No option to square off on your own.

On expiry day, settlement will happen based on the settlement price of Nifty.