In most cases, you can change the Subject Line for most marketing pieces quite easily.
First, make sure you're on the Create Marketing Piece | Edit page. In the middle of the page, you should see a Subject Line box. Click inside the box and enter your desired text. Click the Save button to save your new subject line.
Tip: Avoid practices like including multiple exclamation marks (!!!), typing in all caps, and using other spammy language in your subject line.
If you don't see the options mentioned above, you may be using a Static template that has a locked, unchangeable subject line.