Can I edit and resend an invoice with an amended amount using the same invoice number?

Technically - yes - just edit the invoice, save & resend.

Make sure that the customer destoys/deletes the original one.

If the invoice is in a previous VAT period - it will cause problems.

Larger companies close their books pretty promptly each month - so won't be able to delete the original.

In these case, raise/send a Credit Note & reinvoice.

Hope this helps.