Regarding the cancelled bill

dear all...

i have issued a bill and gave the detail of the particular bill in b2b section in gstr 1 for the month of aug 2018 as it was sale to registered dealer... the bill is still with me and subsequently i wanted to cancel the bill what should i do...? as i already filed my gstr 1 for aug... so there will be itc appearing in gstr 2a for party.... how can i cancel the bill?