I cannot change a customised form when using QuickBooks via safari on an iPad

Glad to have you here in the Community, James.

Let's perform some troubleshooting steps so you can customise forms in QuickBooks Online (QBO).

Before we start, did you receive any error message upon trying to customise a form? Any additional information would be greatly appreciated.

In the meantime, let's log in to your QuickBooks Online (QBO) account on your iPad using a private browser. There are times when the browser stores frequently accessed data, thus causing QBO to act weirdly. 

Once logged in, go back and customise the form again. If this works, you need to clear the browser's cache so the system can start fresh. If you get the same result while using a private browser, I suggest switching to a different one.

Just in case you want to view all your sales transactions in QBO, you can check out this article for the detailed steps and information: View sales transactions in QuickBooks Online.

Come back to this post if you have other concerns or follow-up questions about customising forms. I'll be around to provide further assistance.