How to get a value in another table, store it in a variable or data reader, and then use it to update another table
SqlConnection conUpdate = new SqlConnection(GetConnectionString());
    conUpdate.Open();

    SqlCommand com2 = new SqlCommand();
    com2.Connection = conUpdate;
    com2.CommandText = "SELECT Students.StudentID, Users.UserID FROM Students, Users " +
        "WHERE Students.UserID = Users.UserID";

    int UserId = ((int)com2.ExecuteScalar());

    com2.CommandText = "SELECT MAX(StudentID) FROM Students";

    int StudentId = ((int)com2.ExecuteScalar());

    com2.CommandType = CommandType.Text;
    com2.CommandText = "UPDATE Users SET UserName=@UserName, Password=@Password WHERE UserID=@UserID";

    com2.Parameters.Add("@UserName", SqlDbType.NVarChar);
    com2.Parameters.Add("@Password", SqlDbType.NVarChar);

    com2.Parameters[0].Value = reader;
    com2.Parameters[1].Value = reader;

    com2.ExecuteNonQuery();
    conUpdate.Close();
    conUpdate.Dispose();

SqlDataReader reader = com2.ExecuteReader();
while (reader.Read())
{
     int UserId = Convert.ToInt(reader[0]);// or reader["UserID"]
}
reader.Close();

SqlDataAdapter a = new SqlDataAdapter(com2, connection);
DataTable dt = new DataTable();
a.Fill(dt);

// Execute the query
SqlDataReader rdr = cmd.ExecuteReader();
int UserId;
while(rdr.Read())
{
  UserId = Convert.ToInt32(rdr["UserID"].ToString());
}

SqlConnection conUpdate = new SqlConnection(GetConnectionString());
    conUpdate.Open();

    SqlCommand com2 = new SqlCommand();
    com2.Connection = conUpdate;
    com2.CommandType = CommandType.Text;
    com2.CommandText = "SELECT Students.StudentID, Users.UserID FROM Students, Users " +
        "WHERE Students.UserID = Users.UserID";

    SqlDataReader reader = com2.ExecuteReader();

    if(reader != null)
    {
        while(reader.Read())
        {
            SqlCommand com3 = new SqlCommand();
            com3.Connection = conUpdate;
            com3.CommandType = CommandType.Text;
            com3.CommandText = "UPDATE Users SET UserName=@UserName, Password=@Password WHERE UserID=@UserID";
            // Assuming that you need both the UserName and Password to default to StudentID
            com3.Parameters.AddWithValue("@UserName", reader.GetString(0)); // Assuming StudentID is NVARCHAR
            com3.Parameters.AddWithValue("@Password", reader.GetString(0)); // Assuming StudentID is NVARCHAR
            com3.Parameters.AddWithValue("@UserID", reader.GetString(1)); // Assuming UserID is NVARCHAR
            com3.ExecuteNonQuery();
        }
        reader.Close();
    }

    conUpdate.Close();