For generated emails, you can edit the subject line to whatever you want.
Specifically, for Promo emails, you can edit much more, such as:
1. Segment - This is where you decide who you want to receive your Promotional offer. You have three options between Everyone, Buyers, and Non-buyers.
2. Discount Type - You can select from No Discount, $ Discount, and % Discount.
3. Discount %/$ Off - Here is where you decide how much of a discount, based off type, you want to offer in this Promotional email.
4. Minimum Purchase Amount - This is the threshold your customer must purchase in $ before a discount can be applied. This discount can be any amount, ranging from $0 - $1,000,000.
5. Promotion Start/End date and time - This is the window when your Promotion will be active. Soon after your promotion start time, your Promotion email will send. As soon as your Promotion ends, the discount codes you sent out will no longer work on any purchase.
Here's where you edit all that information:
To get to that page, click the "Advanced" button when you're in your editor view of your email: