I had invoices set to bill based on hours rather than quantity, however, something recently changed/invoices are set back to quantity. Can't find how to change it back.

Thanks for reaching out to this forum, twise1. Let me help ensure invoices are set back to bill based on hours in your QuickBooks Online account.

If you're referring to the billable time by job feature in your company settings, you can verify this option to make sure invoices are billed based on hours. To do this:

1.  Go to the Gear icon and select Account and Settings.
2. Click the Time tab.
3. Turn on the Allow time to be billable setting. 
4. If you want your users to see their billable rate when they enter their time, select the Show billing rate to users entering time checkbox. You may want to leave this unchecked if you bill customers at a different hourly rate than you pay your employees and subcontractors.
5. Once done, select Save, then Done.

If you didn't make these changes, you can review your company's audit logs to check who made the alterations. This way, you'll always keep track of all account activities in your file.

In addition to that, if the column name was modified to Quantity instead of Hours, you can review this setup on your custom form styles. Please refer to these steps:

1. Go to the Gear icon and click Custom Form Styles. Then, select the specific invoice format.
2. Under the Content tab, select the center section and click Edit labels and widths.
3. From the Quantity checkbox, change the name column to Hours.
4. Review all other details and click Done.

Moreover, you can run specific reports to help review your business finances and other accounting data.

I'll keep an eye on this thread if there's anything else you need further assistance with when managing invoices in your account. Let me know any time, and I'll make sure to get you covered. Keep safe!