Insert/Update

With the code below what would be the best way to check to see if "Username" is allready in the database... if it is updateit if not insert it?

  Public Shared Sub trackusers(ByVal thepage As String)

        Using con As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Documents and Settings\perkinj\My Documents\Visual Studio 2010\WebSites\runningprofiles\forums\forum.mdb")
            Dim Username As String = Membership.GetUser.UserName.ToString
            Using cmd As New OleDbCommand("INSERT into nousersonline (Username,Activity,PageUrl,ActivityDate) VALUES ('" & Username & "','', '" & thepage & "','" & DateAndTime.Now.ToString & "')", con)
                con.Open()
                cmd.ExecuteNonQuery()
                con.Close()
            End Using
        End Using
    End Sub

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