Dear HeatherChloe,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
As far as I know, Excel takes that information from the system settings. You have to change the system-wide decimal separator in System Preferences.
You may go to System Preference>Language & Region>Advanced>General>choose dot for the two options for Decimal>OK>Restart your Mac and check the result in Excel.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad