3B related

It appears that you have been inducted into QRMP scheme by default. As a QRMP taxpayer, you are required to file IFF (Invoice furnishing facility) for Jan & Feb and file GSTR 3B for Mar. You have the option to pay your taxes for 1st two months either by paying 35% of taxes paid in last quarter or by paying your self assessed tax for each month through PMT 06.

Thank you sir

You don't have to do anything for the month of January since you are under quarterly filing system and don't have any liability

If your turnover is less than 5 crore then QRMP scheme is applicable to you from 1st Jan. This is why GSTR-3B is not appearing now. An option was given to all taxpayers with turnover less than 5 crores to select GSTR-1 and GSTR-3B monthly filing. If you have not selected it then GSTR-3B will be quarterly for you. If you want to change this then you can do this at the end of this quarter. In the current month since there is not liability to be paid by you it is ok if you do not fill the challan and file PMT-06

Can I change monthy return from April?

turnover below 2 crore sir

Yes you can opt for monthly return