This is the update method in my class

        public function update() {

            $query = 'UPDATE ' . $this->table . ' SET grade = :grade WHERE id = :id';

            $stmt = $this->conn->prepare($query);

            $this->id = htmlspecialchars(strip_tags($this->id));
            $this->grade = htmlspecialchars(strip_tags($this->grade));

            $stmt->bindParam(':id', $this->id);
            $stmt->bindParam(':grade', $this->grade);

            if($stmt->execute()) {
                return true;
            }

            printf("Error: %s.\n", $stmt->error);

            return false;
        }

This is my update api file.

<?php
    /**
    * Phoenix Hospitality Management System
    * PHP Script that creates a new grades in the database
    * Copyright (c) Troy Marker Enterprises
    *
    * @copyright Copyright (c) Troy Marker Enterprises
    * @link
    * @since     1.0.0
    * @license
    */
    header('Access-Control-Allow-Origin: *');
    header('Content-Type: application/json');
    header('Access-Control-Allow-Methods: PUT');
    header('Access-Control-Allow-Headers: Access-Control-Allow-Headers,Content-Type,Access-Control-Allow-Methods, Authorization, X-Requested-With');

    include_once '../config/Database.php';
    include_once '../models/Grades.php';

    $database = new Database();
    $db = $database->connect();

    $grade = new Grade($db);

    $incoming_id = isset($_POST['id']) ? $_POST['id'] : die("API ERROR: Missing grade id parameter");
    $incoming_grade = isset($_POST['grade']) ? $_POST['grade'] : die("API ERROR: Missing grade name parameter");

    if($grade->update()) {
        echo json_encode(array('message' => 'Grade Updated'));
    } else {
        echo json_encode(array('message' => 'Grade Not Updated'));
    }