Select and delete contents below diagonal of cell array

to start from active cell, try the below
the size of the relevant array (6X6, where n=6) is at the top of the code but you can change this as you like

Sub below_diag2()
Const n As Long = 6
With ActiveCell.Resize(n, n)
    a = .Value
    For i = 1 To n
        For j = 1 To i - 1
             a(i, j) = vbNullString
        Next j
    Next i
    .Value = a
End With
End Sub