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