In May Month There is no B2Cs Sale but there is credit note related to B2Cs Sale of March Month.Where this should be shown in GSTR-1 .

Reply—Details of credit or debit notes or refund vouchers issued to unregistered persons is required to be given in Table 9B "Credit / Debit Notes (Unregistered)" of GSTR-1.

Credit or debit note of B2C large invoices can be declared in above tab. In your case if it is not a B2C large invoice taxpayer can amend the figures of B2C supply in Table 10 of GSTR-1.