If you are using the Preview of the Azure Functions Runtime 2.0, it's about to get an update. This GitHub link tells you how to pin any of your Function Apps to the current version, so they won't get automatically upgraded.
This will give you time to separately test if your app will run without issues in the new version, once it's released.
Any unpinned apps will have their Runtime environment automatically upgraded.
https://github.com/Azure/app-service-announcements/issues/129
