Any QuickBooks Online subscribers can customize the invoice template. For the starting, you required to check the invoice window and there is a popup menu on top where you can select from the templates or edit the current one.
For customizing your invoice template you can make a new template and style it the way you like.

Start the following instruction

To start you need to tap the Gear icon beneath your company name and then click on custom form styles. Then tap on the new style icon in the top right corner and choose Invoice. Here you find a design icon where you can change logo, name, font, color, and margins. Now tap the Content, for customizing the header, footer, the body of the invoice’s template and once you finished click done.

For more details, you can check this video about how to customize invoices, estimates, and sales receipts. >> https://quickbooks.intuit.com/learn-support/en-us/customize-forms/customize-your-invoices-estimates-and-sales-receipts-in/01/186093
In case of occurring any problem or you are not able to do the required changes, try to get in touch with the customer care team and add your details to the list of affected users. This will help you to receive updates on the investigation through email.
Also, you can contact Phone support by clicking this link;https://help.quickbooks.intuit.com/en_US/contact.