To edit your email subject lines & preheader text within an Automation flow, you'll first want to go to Automation > Dashboard and then click on the Flow Name in the Automations table.
From there, you'll want to locate the Action Step that you are wanting to make changes to. Click on that step and you will see an email settings panel appear on the left side of the screen.
After clicking on the Action Step, you should see the ability to edit multiple items on the lefthand side - including editing your subject line & preheader text.
Once you are done editing, click on the Save and Exit Button in the top right corner.