Update method dont work
Try property indenting and surrounding your code in code blocks. Please take the extra minute to ensure your code is properly formatted. I've done it for you this time as a courtesy. There is something wrong in your code or in the state of your database, because I get an error (as expected):

```python
Traceback (most recent call last):
  File "x.py", line 25, in <module>
    obj = Lista.get(Lista.cuenta == "test")
  File "/home/charles/pypath/peewee.py", line 4926, in get
    return sq.get()
  File "/home/charles/pypath/peewee.py", line 3185, in get
    % self.sql())
__main__.ListaDoesNotExist: Instance matching query does not exist:
SQL: SELECT "t1"."id", "t1"."cuenta", "t1"."tiempo" FROM "lista" AS t1 WHERE ("t1"."cuenta" = ?)
```
```python
from peewee import *
from playhouse.sqlite_ext import SqliteExtDatabase
import datetime

db = SqliteExtDatabase('lis22.db')

class BaseModel(Model):
    class Meta:
        database = db

class Lista(BaseModel):
    cuenta = CharField(unique=True)
    tiempo = CharField()

try:
    db.connect()
    db.create_tables([Lista])
except:
    pass

lista = Lista.create(cuenta="test", tiempo="a")
a = Lista.update(cuenta="test_changed").where(Lista.cuenta == "test")
a.execute()
Lista.get(Lista.cuenta == "test").cuenta

#output:  u'test'
```