You can use the Server app to change an account’s full name, but you can’t change its short name.
Names can be up to 255 characters (or up to 85 variable-width encoded characters), and can include spaces.
Select Users in the Server app sidebar, then do one of the following:
To change a user’s name, double-click a user.
To change a group’s name, double-click a group.
Enter the new name in the Full Name field, then click Done.