When your Client Secret is expired, you have to renew it in Microsoft Entra Admin Center. You do this by executing the following steps.
Go to Microsoft Entra Admin center.
Click on Applications, next go to App Registrations. Click on the File 365 app you created during installation.
In the left hand menu, go to Certificates & secrets. Create a new Client secret.
Add a description of your choosing. Add a expiry period.
Click on the blue “Add” button.
Copy the value of the newly created secret. Make sure you copy the Value and not the ID.
Next, go to Salesforce as an administrator. In Setup, find Auth. Providers in the Quick find menu.
Open the File365 Auth. Provider. Change the Consumer secret with the value you copied from the secret you created in the Microsoft admin panel. Don’t change any other value.
Save and close. Now File 365 should be working correctly and you should be able to use all features of File 365.