public void update(int userId, String date, String column, String value){
    try {
        // convert date from String to Date
        DateTime dt = DateTime.parse(date);
        java.sql.Date sqlDate = new java.sql.Date(dt.getMillis());
        // create prepared statement
        conn = DriverManager.getConnection("jdbc:mysql://localhost/db","root", "");
        String query = "UPDATE expenses_income SET ? = ? WHERE userId = ? AND date = CAST(? AS DATETIME);";
        PreparedStatement preparedStmt = conn.prepareStatement(query);
        preparedStmt.setString(1, column);
        preparedStmt.setString(2, value);
        preparedStmt.setInt(3, userId);
        preparedStmt.setDate(4, sqlDate);
        preparedStmt.executeUpdate();

        conn.close();
    } catch (Exception e) {
        System.err.println("MySQL exception: " + e);
    }
}

MySQL exception: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ''comment' = '123' WHERE userId = 1 AND date = CAST('2018-01-06' AS DATETIME)' at line 1

String query = "UPDATE expenses_income SET ? = ? WHERE userId = ? AND date = ?;";