I believe to do this, you will select the ‘mix’ button under the aux you want to change. Then go to the source strip and click on the button at the top of the strip you labeled ‘post’ and it will change to ‘pre’. I hope thats what you were looking for.

With the current version of Editor software you cannot change the Aux source setting using just the PC. This function will become available with a new release of Editor software.

For now, you need to use a surface to change the Aux source. This is done using the ROUTING screen and selecting the MIX key of the Aux master strip. Select the source from the drop down menu on the right of the screen. This setting applies to all sends to that Aux. You can have different settings for the different Aux mixes.

The Aux source is not affected by the setting of the global Direct Output option.
Hope that helps,
Carey