I love that you're using the QuickBooks Online app in addition to accessing the software from your internet browser. Both the desktop app and the mobile app make it even easier to use QuickBooks Online to manage the books for your business. Let me give you a hand with changing the terms while using the app.
@LeithG has provided you with the steps to change terms if you're using QuickBooks Online in the desktop app or your internet browser. If you're using the mobile app, you can open an Invoice and hit the Edit pencil to make changes. From there you'll want to select the Terms option and choose from the dropdown what the terms for the transaction should be.
If you want to change the terms for a customer, navigate to the Customers page, click on the one you need to adjust, and hit Details. Click the Edit pencil and locate the Terms section to make changes. Choose the payment terms for this specific customer from the dropdown. Hit Save when you've finished.
This is a simple process, but let me know if you have any other questions. I'd love to help in any way that I can. Enjoy your week!