How to update model when data are stored and modified outside the model?

[anda_skoa]

I used to consider a model as an interface instead of a wrapper.anyway.

That is a valid assumption, actually the very goal of most models.

You only need to make sure the model knows about changes to the actual data, but that can be done in various ways.
For example the class modifying the data could emit signals, the data could be in a class that has signals, the modifying code could tell the model, or, as wysota already said, the model can the only interface to the data.

Cheers,
_