PHP MYSQLI not updating : So I have been trying to make a form where I can update two fields one field is going be admin_welcomebox and admin_author and I'm trying update it by the id so here go my code

							<div class="col-lg-6">						
								<div class="panel panel-color panel-inverse">								
									<div class="panel-heading">
										<h3 class="panel-title">Welcome Box Update</h3>
									</div>	
									
									<?php
									if(isset($_POST["submit"])){
										$servername = "localhost";
										$username = "trres";
										$password = "sss";
										$dbname = "txxxs";

										// Create connection
										$conn = new mysqli($servername, $username, $password, $dbname);
										// Check connection
										if ($conn->connect_error) {
											die("Connection failed: " . $conn->connect_error);
											}

										$sql = "UPDATE admin_news SET welcomebox = '{$admin_news}' SET author = {$admin_author} id='{$id}'";

										if ($conn->query($sql) === TRUE) {
										echo "<h4 class='bg-success'>You have updated admin welcome box.</h4>";
										} else {
										echo "<script type= 'text/javascript'>alert('Error: " . $sql . "<br>" . $conn->error."');</script>";
										   }
										  $conn->close();
										  }
										?>	
									<div class="panel-body">	
									
									<form method="post" action="">
										<div class="form-group">
											<label for="welcomebox">Welcome Box</label>
											<textarea type="text" name="welcomebox" id="welcomebox" placeholder="Enter Your Message" class="form-control"></textarea>
										</div>		
										<div class="form-group">
											<label for="author">Author Name</label>
											<input type="text" name="author" id="author" placeholder="Author Name" class="form-control" / >
										</div>											
										<div class="form-group text-right m-b-0">
											<button class="btn btn-primary waves-effect waves-light" type="submit" name="submit" id="submit">
												Update Info
											</button>
										</div>
										
									</form>									
									</div>												
								</div>								
							</div> 

When i try update it just refresh the page nothing else.