GSTR3B is only a summary return for payment of tax. It is not concerned with an invoice being B2B or B2C. The details of such invoices are to be given in GSTR1. Incase you have not filed the GSTR1 for the respective month, you may make such change.
If you not filed GSTR1, then you can change B2C to B2B. Note: while change To B2B bills be careful that the other existing B2B invoice number should not change.
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