It is not quite the same as Excel, but as Greg suggests it is just a matter of setting a range to the place you want to process. e.g.
Code:
Sub Macro1()
Dim oTable As Table
Dim oCell As Range
Set oTable = ActiveDocument.Tables(1) 'where 1 is the first table
'or
'Set oTable = Selection.Tables(1) 'which is the table the cursor is in
Set oCell = oTable.Rows(1).Cells(1).Range 'the first cell in the top row
oCell.End = oCell.End - 1 'omit the cell end character from the range
oCell.Text = UCase(oCell.Text) 'make the cell range upper case
Set oCell = Nothing
Set oTable = Nothing
End Sub