You must have "Full Access" or "Limited Access - Financials" permissions to edit a payment plan.
NOTE: The total balance must remain the same when editing a payment plan in Invoicing. If you want to reduce the balance, you should apply a credit to the invoice.
From the left navigation menu, click Finances > Invoicing or Financials > Orders.
Find the invoice or order you want to credit and click the Sale ID.
To the right of the Payment Plan, click the Pencil Icon.
Change each payment's installment dates/amount, ensuring the total balance remains the same.
When finished, click Save.