Hey there suzannecfournier,
Making sure QuickBooks Online is displaying the information you need on your forms is an important part of setting up your program. Likewise, it's also good to be able to make edits to or even remove items when they no longer apply to your business. I can guide you to removing your HST number from your invoices.
Your sales tax numbers are tied to the sales tax setup in the program, so when you want to make changes, going to the sales tax centre will allow you to do so. Here are the steps to take.
Keep in mind this will remove the number from all of your sales forms, not only the invoices.
Let me know if you have further questions about that. Have a great day.