This could be done in a couple of ways as below:
Reverse the Original Transaction
Go into the Session Postings and pick the entry you have posted by selecting Purchase in the radio button.
Select the Reverse option and OK.
If you want to re-post the transaction then you can select the option Do you want to amend and re-post this posting and fill in the details and click OK.
📌Note: You won't be able to reverse an invoice if the payment has been matched, this will need to be unmatched first by doing a minus posting when selecting the Match.
Post a Purchase Ledger Credit note
Select the Supplier tab.
Select Supplier Credits.
Click on Search unless supplier is known and select one.
Fill in the details of the credit.
Pick the Invoices button and select an invoice for it to go against.
Go into the Costs button and VAT and select the apportionment.
Select OK/Next.