To uninstall Fiori apps during an SAP S/4HANA system conversion, you can follow these necessary steps:

1. Identify the Fiori apps: Determine which Fiori apps you want to uninstall from your SAP S/4HANA system. Make a list of the app names or IDs that you wish to remove.

2. Check dependencies: Before uninstalling any Fiori app, it's important to check if there are any dependencies on other apps or components. Some Fiori apps may have dependencies on backend services, business processes, or other apps. Ensure that removing the app will not impact the functionality of other components.

3. Disable the Fiori app: Once you have identified the Fiori app to uninstall, disable it in the SAP Fiori Launchpad Designer. Access the Fiori Launchpad Designer and navigate to the app you want to uninstall. Disable the app by removing it from the relevant catalogs or groups.

4. Remove app-specific configurations: Some Fiori apps may have specific configurations or settings that need to be removed. Check if there are any app-specific configurations in the SAP Fiori Launchpad Designer or other relevant areas, and remove them accordingly.

5. Delete app-specific data: If the Fiori app has generated any specific data or entries in the system, such as custom tables or entries in standard tables, you may need to delete them. Identify the relevant data associated with the app and delete it using appropriate SAP transactions or tools.

6. Transport changes: If you have made any changes to the Fiori app or its configurations, ensure that these changes are transported to other systems as necessary. Follow the standard transport process to move the changes to the desired systems.