How can I change quantity (QTY) to CASES when entering a suppliers bill?

Good day, @charmainemiller9.

The option to change QTY to cases in the Bill window is unavailable in QuickBooks Online. This section is a default interface of the program and can't be customized. 

I can see how this option would help you in tracking your goods. Therefore, I'll take note of this as a suggestion to improve your experience. 

On the other hand, you can personally send your suggestions to our developers. They can review your request and most likely include them in future updates. Here's how:
 

1. Select Settings and click Feedback.
2. Enter your comments or product suggestions. Then select Next to submit feedback.

For additional reference in recording bills and bill payments in QuickBooks, please check this article: How to track bills and record the payments in QBO.

If you have further queries about tracking your goods and services in the program, just add them to the thread and I'll help you out. Have a good one.