How do I update Drupal 7 to Drupal 7 DEV via Drush command line? I have an old drupal 7 installation and I would like to update this via drush. Usually I download the drupal 7 dev version through the website using wget and untar it into the public_html directory and overwrite everything. 

I know i can update drupal modules via "drush dl " but not sure how to do it for core easily using drush.
drush dl is for downloading modules the first time, not updating.

To update core to dev version:

drush up drupal-7.x-dev