Gst export invoice uploaded as with tax instead of without tax

I uploaded an export invoice of January 2018 as with payment of tax instead of without payment of tax although we are a 100% export unit under LUT. I still haven't filed the GSTR1 returns April 2018 onwards. Can I amend the invoice to without payment of tax in the April return? If yes, what should I enter in the revised invoice number? Or do I pass a credit note against that invoice, and then upload a new one under without payment of tax? Since I export only services, the port and shipping code, etc., are not involved. Thank you.