If you are a customer of the State Bank of India, then you need to apply for a new cheque book by March 31. The chequebook you were using till now will be invalid from April 1.
After the merger, all the customers of the core banks of the State Bank are now customers of the SBI. Thus, if you still have the chequebooks of Bharatiya Mahila Bank, State Bank of Mysore, Bank of Bikaner, Bank of Hyderabad, Bank of Patiala and others, you must apply for a new cheque book within March 31.
You can apply through internet banking, mobile banking, ATM or can visit the bank and apply for the new chequebook.
Here is the Tweet by the SBI and the video posted by the State Bank of India: