if (isset($_POST['password_change'])) {
    $hash = password_hash($password);
    $username = strip_tags($_POST['username']);
    $password = strip_tags($_POST['old_password']);
    $old_password = $hash;
    $newpassword = strip_tags($_POST['new_password']);
    $new_password = $hash;
    $confirmnewpassword = strip_tags($_POST['con_newpassword']);
    $con_newpassword = $hash;

    $stmtUsers=$con->prepare("SELECT COUNT(*) FROM users where username=? limit 1");
    $stmtUsers->bind_param("s",$username);

    if($stmtUsers->execute()) {
        $hash = $stmtUsers->fetch();
        if ($password == $hash['password']){
            if($newpassword == $confirmnewpassword) {
                $stmtUpdate=$con->prepare("UPDATE `users` SET `password` = ? WHERE `username` = ?");
                $stmtUpdate->bind_param("ss",$newpassword,$username);
                if($stmtUpdate->execute()){
                    echo "¡Contraseña cambiada con éxito!";
                } else{
                    echo "La contraseña no se pudo actualizar";
                }
            } else {
                echo "¡Las contraseñas no coinciden!";
            }

        } else {
            echo "Por favor, escriba su contraseña actual con precisión!";

        }
    } else {
        echo "Nombre de usuario incorrecto";
    }
}

<form name="resetform" action="changepass.php" id="resetform" class="passform" method="post" role="form">
    <h3>Change Your Password</h3>
    <br />
    <input type="hidden" name="username" value="<?php echo $sname; ?>" ></input>
    <label>Enter Old Password</label>
    <input type="password" class="form-control" name="old_password" id="old_password">
    <label>Enter New Password</label>
    <input type="password" class="form-control" name="new_password" id="new_password">
    <label>Confirm New Password</label>
    <input type="password" class="form-control"  name="con_newpassword"  id="con_newpassword" />
    <br>
    <input type="submit" class="btn btn-warning" name="password_change" id="submit_btn" value="Change Password" />
</form>

<!--display success/error message-->
<div id="message"></div>

<script type="text/javascript">
$(document).ready(function() {
    var frm = $('#resetform');
    frm.submit(function(e){
        e.preventDefault();

        var formData = frm.serialize();
        formData += '&' + $('#submit_btn').attr('name') + '=' + $('#submit_btn').attr('value');
        $.ajax({
            type: frm.attr('method'),
            url: frm.attr('action'),
            data: formData,
            success: function(data){
                $('#message').html(data).delay(3000).fadeOut(3000);
            },
            error: function(jqXHR, textStatus, errorThrown) {
                $('#message').html(textStatus).delay(2000).fadeOut(2000);
            }

        });
    });
});
</script>