By pure chance did I find this, and after my modification, and it seems to work
ui->tblView->model()->setData(ui->tblView->model()->index(1,4,QModelIndex()), QString("Testar"), Qt::EditRole);

So if I got this right, since I set my QTableView Model to the proxymodel and filter with. The above tblView->model() returns the result from the filter as a model....
Also, using ->setData let min change the data as long as I'm using the QStandardItem in my model plus using Qt::EditRole.