Whenever desired, you can easily edit a saved invoice to remove discount applied - in as much as the invoice hasn't been sent to and paid by the client. Such removal can be done for both discount applied to the invoice's total amount or individual line items.
Removing a discount applied to individual line items
To remove a discount from invoice:
Navigate to Invoices, then tap Edit while hovering on your desired invoice
Go to each item's Discount field and clear the figures earlier inputted
Tap Save to wrap it up
Removing a discount applied to the total amount
To remove a discount applied to an invoice's total amount:
Scroll down to the end of the invoice, then clear the Invoice discount field
Scroll back, then tap Save to save your edit