How do I fix the error when updating information in a MySQL table?

I'm having some problems trying to work out how to update a MySql table with my php code. This is the section so far, the code should either update the table or add a new column depending on weather an new column has already been made in the database on that date.

Edit: this is a lot of code, just to give context to what I am trying to do, the part of code throwing the error is shown separately below as well :)

  $sql = "SELECT * FROM $username WHERE day=?;";
  // Here we initialize a new statement by connecting to the database (dbh.php file)
  $stmt = mysqli_stmt_init($conn);
  if (!mysqli_stmt_prepare($stmt, $sql)) {
    // If there is an error the user is sent to the enter data page again
    header("Location: ../enterTodaysData.php?error=sqlerror");
    exit();
  }
  else { //if there are no errors...
    mysqli_stmt_bind_param($stmt, "s", $day); //binds the parameters to the statement
    mysqli_stmt_execute($stmt); //executes the statement

    $result = mysqli_stmt_get_result($stmt); //saves the result of the statement into the result variable

    if ($row = mysqli_fetch_assoc($result)) { //if the user HAS already made an entry that day
      $sql = "UPDATE $username SET (peakflow1, peakflow2, coughing, tightChest, shortBreath, wheezing, symptomOne, symptomTwo, medication, mood, comments, overall WHERE day) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?);";
      $stmt = mysqli_stmt_init($conn);
      if (!mysqli_stmt_prepare($stmt, $sql)) {
        // If there is an error the user is sent to the enter data page again
        header("Location: ../enterTodaysData.php?error=sqlerror");
        exit();
      }
      else { //if there are no errors...
        mysqli_stmt_bind_param($stmt, "iisiiiiiiiiss",  $peakflow1, $peakflow2, $coughing,  $tightChest,    $shortBreath,   $wheezing,  $symptomOne,    $symptomTwo,    $medication,    $mood,  $comments,  $overall, $day);
        mysqli_stmt_execute($stmt); //executes the statement

        echo "<script type='text/javascript'>alert('Data entered successfully!');</script>";
        header("Location: ../home.php?sql=success");
        exit();
      }
    }
    else{ //if the user has not
      $sql = "INSERT INTO $username (day,   peakflow1,  peakflow2,  medication, mood,   coughing,   tightChest, shortBreath,    wheezing,   symptomOne, symptomTwo, overall,    comments) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?);"; //the question marks are placeholders
      $stmt = mysqli_stmt_init($conn);
      //an sql statement is prepared and the database is connected to
      if (!mysqli_stmt_prepare($stmt, $sql)) {
        // If there is an error the user is sent back to the signup page
        header("Location: ../enterTodaysdata.php?error=sqlerror");
        exit();
      }
      else {
        //binds the paramaters and data to the statement
        mysqli_stmt_bind_param($stmt, "siisiiiiiiiis", $day,    $peakflow1, $peakflow2, $medication,    $mood,  $coughing,  $tightChest,    $shortBreath,   $wheezing,  $symptomOne,    $symptomTwo,    $overall,   $comments);
        //this executes the prepared statement and send it to the database, this registers the user.
        mysqli_stmt_execute($stmt);
        //sends the user back to the signup page, with a message confirming that it was a success
        echo "<script type='text/javascript'>alert('Data entered successfully!');</script>";
        header("Location: ../home.php?sql=success");
        exit();
      }
    }
  }

This is the part of code that the error is coming from:

  $sql = "UPDATE $username SET (peakflow1, peakflow2, coughing, tightChest, shortBreath, wheezing, symptomOne, symptomTwo, medication, mood, comments, overall WHERE day) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?);";
  $stmt = mysqli_stmt_init($conn);
  if (!mysqli_stmt_prepare($stmt, $sql)) {

This is the error I am currently getting:

Fatal error: Uncaught mysqli_sql_exception: 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 '(peakflow1, peakflow2, coughing, tightChest, shortBreath, wheezing, symptomOne, ' at line 1 in C:\Users\MMRUD\Documents\XAMPP\htdocs\AsthmaAssistant\php_code\todaysdata.php:47 Stack trace: #0 C:\Users\MMRUD\Documents\XAMPP\htdocs\AsthmaAssistant\php_code\todaysdata.php(47): mysqli_stmt_prepare(Object(mysqli_stmt), 'UPDATE test SET...') #1 {main} thrown in C:\Users\MMRUD\Documents\XAMPP\htdocs\AsthmaAssistant\php_code\todaysdata.php on line 47