Having the fan blow out will mostly repeatedly replace the air by the window, as the air sucked by the fan is taken from the immediate surroundings, and that air is standing mostly still.
If you let the fan blow into the room however, it will be pushed further into the room, helping to move the air in the room more, and increasing how much air is actually shifted out.
The problem with taking in the air it blows from the immediate vicinity still exist, but normally there will be some movement in outside air, so the that is sucked fr4om the immediate vicinity is replaced by the natural wind replacing it outside.

So blowing in replaces way more air on average, the exception being if it works against an airflow in the room in opposite direction.