Good day, @Cathal_Reynolds,
I want to make sure about the rule you're referring to. Any clarification will help me sort things out.
In the meantime, if you're referring to Bank Rules, follow the steps below to modify them:
See this link for more information about this: Setup bank rules to categorise online banking transactions in QuickBooks Online
Also, are you using QuickBooks Online for USA? There are several rulings you can do for this version, namely, Price Rules for customers or the Tags you add to invoices.
While waiting for your confirmation, I'm adding some steps that might work for QBO USA users only.
To edit or delete Price Rules:
Here is an article about Price Rules: Set price rules in QuickBooks Online
To modify tags on invoices, go to the Gear icon choose Tags. Tap the drop-down in the Action column then choose Edit Tag or Delete Tag.
To learn more about this feature, see the following links:
If you're referring to something else, let me know in the comment below. I'll be right here if you need anything else. Have a good one!