If you mean the Windows Mail title at the top of the window, go to the
Windows Orb (Start), type regedit, press Enter and navigate to registry key
HKEY_CURRENT_USER\Software\Microsoft\Windows Mail
Look for WindowTitle (no space) then right-click it then Modify to change
it. Alternatively, if it doesn’t exist click Edit > New > String Value and
name it WindowTitle, right-click it and type the title you want.
To additionally remove the annoying splash screen, go to the Windows Orb
(Start), type regedit, press Enter and navigate to registry key
HKEY_CURRENT_USER\Software\Microsoft\Windows Mail then click Edit > New >
DWORD Value and name it NoSplash, right-click the name and give it a value of
1.