HI @dtd646 · Thank you for reaching out.
Modification to synced users via Azure AD can only be done after Turning off directory synchronization by using below cmdlet:
Set-MsolDirSyncEnabled -EnableDirSync $false
Keep in mind that after directory sync is disabled, no changes can be synced from On-premises AD to Azure AD and re-enabling the directory sync may override the changes that you have done while directory sync was disabled.
If you don't want to turn off directory sync, the changes to synced objects can only be done via on-premises AD and then syncing the change to Azure AD.
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