class User(db.Model):
    name = db.StringProperty()
    password = db.StringProperty()

user = User.all().filter('name =', name).filter('password =', password).get()
if user:
    user.delete()

user = User.all().filter('name =', 'John').filter('password =', '1234').get()
if user:
    user.name = 'Marcus'
    user.put()