You can only edit a purchase credit note if it meets the following conditions:

• The credit is unpaid
• The credit has no allocations
• The credit isn't on a VAT Return

If the credit has allocations, remove them before editing it by:

• Delete the refund matched to a credit note, or
• Unallocating the credit note from the purchase invoice

If the credit is on a VAT Return:

• If it's included on a draft VAT Return, delete the VAT Return before editing the invoice
• If it's included on a submitted VAT Return, you can't edit the invoice. Instead, reverse the purchase credit note

1. Go to Purchases, then Purchase credit notes.
2. Find and open the relevant credit note. 
3. Select Edit.
4. Make the required changes and select Save.