Debit Note & Credit Note

Hlo, in GSTR 1 there is a tab for showing credit/debit note. You can provide the details there.
Credit note issued for lowering tax and debit note issued for higher in tax.
While filing return, In case of debit note, you have to add the tax to your taxable value and in case of credit note issue, you have to substrat from taxable value.
Impact of ITC will automatically cover in this procedure.

The credit & Debit Note issued in regard to outward supply are only reported in GSTR 1.

In 3B :
Any credit debit note tax amount need to be reduce/Increase from output (sale) or ITC ( purchase) for that particular month & net of amount need to be reported in GSTR 3B