Most processes behind soft crediting a payment require the payment's batch to still be unapproved. However, you can still apply a soft credit directly to an individual payment, even if its batch has already been approved.
To do so:
Click Finance, then select Batches.
Find and select the approved batch you want to update.
Click the Payments tab.
Search for the payment you want to adjust.
If there are many payments, use column filters to narrow your search.
Click the eye icon next to the payment.
Click More, then select Create Soft Credit.
Choose the contact you want to receive the soft credit.
When asked to confirm, click Yes.
You’ll see the soft credit displayed under the Soft Credits tab for that payment.