Explained in this post.
After the June expiry, the Nifty long-term options contracts (where the expiry is greater than 3 months) will be revised from the current lot size of 75 to 50. The average closing price of Nifty (cash) will be taken to adjust the contract value. [Alternatively, the exchange may do an M2M settlement for the 25 units being reduced from the lot size. We’ll update this post once we get the confirmation.]