Provided some title has been set, use:
   currentdb.Properties("AppTitle") = "Hello World"
   Application.RefreshTitleBar

If the property does not exist, you need to create it.
The SetPropertyDAO() function in this link will set the property, creating
it if needed:
   http://allenbrowne.com/AppPrintMgtCode.html#SetPropertyDAO

This is code from one of my applications.  Straight from Access Help.  You
can enter this directly into a module.  Pass the variable (in this case
'PilotName') and it should update your title bar.  You can obviously edit the
code to suit your needs.  Hope this helps, and let me know if you have any
questions.

Sub ChangeTitle(PilotName As String)
   Dim obj As Object
   Dim dbs As DAO.Database
   Const conPropNotFoundError = 3270
   
   On Error GoTo ErrorHandler
   ' Return Database object variable pointing to
   ' the current database.
   Set dbs = CurrentDb
   ' Change title bar.
   dbs.Properties!AppTitle = PilotName & " Pilot Log Book"
   ' Update title bar on screen.
   Application.RefreshTitleBar
   Exit Sub
   
ErrorHandler:
   If Err.Number = conPropNotFoundError Then
       Set obj = dbs.CreateProperty("AppTitle", dbText, PilotName & " Pilot
Log Book")
       dbs.Properties.Append obj
   Else
       MsgBox "Error: " & Err.Number & vbCrLf & Err.Description
   End If
   Resume Next
End Sub