You need to turn-off the "Auto Generation Of Invoice Number" to edit the invoice numbers in the drafted invoices. To do this, you could get into the 'New Invoice' page. There you could click the "Change Settings" option for the invoice number and can click the "No" option for the auto-generation. Once saved this settings, you could get back and edit the drafted invoice where you can manually update/change the invoice number.
Zoho Invoice FAQ