gstr4 entry correction

i have filed my gstr4 without purchase details entry as last two quarter and submiited 1% turnover tax.
now it says 4A,4B mandatory.
is there any provsion of re entering Purchase details
later after 18th april last date.it seems each entry has to be made manually for each tax slab for one single invoice. and process seems to be tiring.seems gst has made simple and hazzlefree composition to big complicated affair.