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You can utilize the discount function depending on the status of your invoice. If it's not yet paid, you turn it on from the Gear and enter the discount amount.
But if your invoices are closed and paid already, you'll want to delete and recreate the invoice, including the payment, to utilize the discount function. You can read this article to learn more: Add a discount to an invoice or sales receipt in QuickBooks Online.
I'm adding a reference here for steps in case you wanted to customize your invoice template.
I'll be here if you need more help. Thanks!