For Outlook app navigation initials, the supported way to change the letters shown is to rename the account description:
- Open Settings.
- Go to Accounts.
- Select the account.
- Open Description.
- Edit the account name so the sidebar reflects the initials wanted.
The change appears the next time the sidebar is opened.
If the initials are coming from profile or account name data instead, that must be changed in the account profile rather than directly in Outlook for iOS:
- For a personal Microsoft account, change the display name by signing in to the Microsoft account and selecting Edit name, then Save. Name changes can take up to 24 hours to update everywhere.
- For a work or school account, some profile fields are controlled by the organization. If the name or organizational info cannot be edited, the update must be made by the IT admin or the relevant HR/IT department.
- On a Windows device, the displayed user account name depends on the account type:
- Personal account: update the name in Your info for the Microsoft account.
- Work/school account: sign in to the work or school account to update the name, if the organization allows it.
- Local account: open Control Panel > User Accounts > Change your account name.
References:
- How do I change the initials that appear in my app navigation? | Microsoft Support
- How can I update my profile information or change my picture? | Microsoft Support
- Why can't I change all my profile information or organizational info that other people can see on my contact card? | Microsoft Support
- How to change your user account name | Microsoft Support
- How to change your Microsoft account name or address | Microsoft Support