How to get an existing RowVersion value from a table to be used when updating the same record?

I'm learning to update a record in a SQL Server table after checking for concurrency violation using a RowVersion column (timestamp data type). However, I'm not able to extract the existing RowVersion value to a suitable variable type. I get the following error and the relevent code is also mentioned below.

Error:

' UPDATE with concurrency check
Public Sub Update(user As UserModel, rowVer As Byte)

    Dim sql As String = "UPDATE TB_USERS SET " &
                    "FullName = @FullName, " &
                    "Password = @Password, " &
                    "UserType = @UserType, " &
                    "NIC = @NIC, " &
                    "Address = @Address, " &
                    "Telephone = @Telephone, " &
                    "Level = @Level, " &
                    "LoginStatus = @LoginStatus, " &
                    "Locked = @Locked " &
                    "WHERE user_Id = @UserId AND RowVersion = @RowVersion"

    Using con As New SqlConnection(connectionString)
        Using cmd As New SqlCommand(sql, con)
            cmd.Parameters.AddWithValue("@FullName", user.FullName)
            cmd.Parameters.AddWithValue("@Password", user.Password)
            cmd.Parameters.AddWithValue("@UserType", user.UserType)
            cmd.Parameters.AddWithValue("@NIC", user.NIC)
            cmd.Parameters.AddWithValue("@Address", user.Address)
            cmd.Parameters.AddWithValue("@Telephone", user.Telephone)
            cmd.Parameters.AddWithValue("@Level", user.Level)
            cmd.Parameters.AddWithValue("@LoginStatus", user.LoginStatus)
            cmd.Parameters.AddWithValue("@Locked", user.Locked)
            cmd.Parameters.AddWithValue("@UserId", user.Id)
            cmd.Parameters.AddWithValue("@RowVersion", rowVer)

            con.Open()
            Dim rowsAffected = cmd.ExecuteNonQuery()
            MsgBox(rowsAffected)
        End Using
    End Using
End Sub

Private Sub cmdUpdateUserData_Click(sender As Object, e As EventArgs) Handles cmdUpdateUserData.Click

    Dim con As New SqlConnection(connectionString)
    Dim sql As String = "SELECT user_id, RowVersion FROM TB_USERS where user_name= '" & txtNewUserName.Text & "'"
    Dim da As New SqlDataAdapter(sql, con)
    Dim dt As New DataTable()
    da.Fill(dt)

    Dim repo As New UserRepository()
    repo.Update(GetUserFromForm(), dt.Rows(0)("Rowversion"))

End Sub