Hi,
Please note in this forum we mainly discuss questions about Office client, such as Outlook, Word, Excel.
As far as I can tell, on Outlook client it is not possible to edit user's own information in GAL.
On Exchange Online, I'm not sure, I suggest you post the question in Office 365 Community forum for further assistance:
https://community.office365.com/en-us/f
Regards,
Melon Chen
TechNet Community Support
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