In Excel VBA, the Application.DefaultSaveFormat property allows you to get or set the default file format that Excel will use when saving workbooks. This property determines the format that Excel will suggest when a user manually saves a workbook unless they explicitly choose a different format.
You can change the default save format by assigning a new value to Application.DefaultSaveFormat. You can see all possible Values Here
Example
Sub Change_Default_Save_Format()
'Change the Defaule save format to 51 (xlOpenXMLWorkbook)
Application.DefaultSaveFormat = 51
End Sub
You can see the current Default Value
Example
Sub Check_Default_Save_Format()
'Check Current Default Value of Workbook
MsgBox Application.DefaultSaveFormat
'Will return back the number that represents the saved type
End Sub