@Alok Kumar Pandey Thanks for reaching here!
Try Option 2: Update FcmV1 credentials via management plane hub operation
or
Option 3: Update FcmV1 credentials via data plane hub operation
If still facing issue suggest you to register for the office hours here: https://outlook.office365.com/book/AzureNotificationHubsFCMOfficeHours@bookings.microsoft.com/