you cant amend export invoice to B2B invoice directlh except it is SEZ supply.
but you can do below
amend export invoice in next month GSTR-1 showing zero value.
and
add B2B invoice in next month GSTR-1. Please nore system wouldnt accept same Invoice No., hence make small change in Invoice No. like adding . in between
Hope the same is service invoice as the goods export invoice would have got recorded at customs port as well.
If it is service invoice you can make the change in GSTR1 by amending the value as zero and making the change in Books of accounts as there is condition of forex realisation.
Raise separate invoice in B2B section with new invoice number in GSTR1