At its most basic you could use something like:
Code:
Sub Demo()
Application.ScreenUpdating = False
Dim StrAddr As String
StrAddr = Trim(InputBox("What is the Address of cell to update?", "Input Destination"))
On Error GoTo ErrExit
ActiveSheet.Range(StrAddr).Activate
ActiveCell.Value = "=SUM(B6:E6)"
ErrExit:
Application.ScreenUpdating = True
End Sub
Note that I haven't added any code to actually validate the address.