Gstr1 and gstr3b

If the agrregate turnover is below 1.5cr the registrant can opt filing quarterly gstr1 returns

but u have to pay the tax liability to the government monthly only. gstr3B must be filed monthly on or before 20th of every month

The Government has given option to file FORM GSTR-1 "QUARTERLY" to small Taxpayers whose Aggregate Turnover does not exceeds Rs. 1.5 crore. However GST Return in FORM GSTR-3B will be Monthly irrespective of any turnover limit.
GSTR-3B have to be filed every month within 20th of the following month ( for e.g. November GSTR-3B within 20.12.2019 ).

As far as changing of option is concerned , you can change it only at the beginning of the new FY.

Note : Filing Returns and Statements under GST Regime is "mandatory". Even if no business transaction has been held in a particular month, then you have to file NIL RETURN for that particular period.

Regards.

If the Aggregate turnover is up to 1.5 crore per annum than you are liable to file GSTR 1 is Quarterly but GSTR 3B is mandatory to file monthly before due date.

As per Notification 45/2019 CT : 

Whose aggregate turnover is below 1.5 crore is required to file  Qtly GSTR 1.

As per Notification 46/2019 CT 

Whose turnover is above 1.5 crore is required to file GSTR 1 monthly.

 

And GSTR 3B need to file by every taxpayer On monthly basis