How are records updated with SqlAlchemy?

I am developing an application in Python with Flask and SqlAlchemy, for this the basic operations that are carried out are to create a record, modify it and delete it. To create the record with the following code fragment

...

try:
    pp = Persona(ci=ci, ext=ext, primerAp=prap, segundoAp=seap, \
        nombres=nom, fecha_nacimiento=fnac) #instaciamos una clase

    pp.usuario = 'usario23'
    pp.fecha = datetime.now()
    pp.estado = 'V'

    db.session.add(pp)
    db.session.commit()

except(KeyError, TypeError, ValueError):
    raise jsonify(resultado='Error')
return jsonify(resultado='Ok', Persona=pp.aJson())

...

I have searched SqlAlchemy's documentation and can not find a way to update an instance of an object, something like db.session.update (pp) db.session.commit ()

Does anyone know how this is done?