save changes flask sqlalchemy

from flask import Flask, render_template, request, redirect, url_for
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///example.db'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)

@app.route('/')
def index():
    users = User.query.all()
    return render_template('index.html', users=users)

@app.route('/add_user', methods=['POST'])
def add_user():
    username = request.form['username']
    new_user = User(username=username)
    db.session.add(new_user)
    db.session.commit()
    return redirect(url_for('index'))

@app.route('/update_user/<int:user_id>', methods=['POST'])
def update_user(user_id):
    user = User.query.get(user_id)
    new_username = request.form['new_username']
    user.username = new_username
    db.session.commit()
    return redirect(url_for('index'))

@app.route('/delete_user/<int:user_id>')
def delete_user(user_id):
    user = User.query.get(user_id)
    db.session.delete(user)
    db.session.commit()
    return redirect(url_for('index'))

if __name__ == '__main__':
    db.create_all()
    app.run(debug=True)