The subject lines for your notification emails are actually tied to Shopify so they have to be edited in the Shopify Dashboard.
Here are step-by-step instructions on to do so:
1. From your Shopify dashboard, click on "Settings" in the button left hand corner:
2. Click on "Notifications":
3. Once you are in the Notifications section, you can see the different email templates, click on the email that you want to edit.
4. The subject lines will be displayed here. Just remember to click the green "Save" button after you have made any edits! :)