You add a rule action to change an email subject with the 'Insert subject text' rule action.

The subject rule action will insert text before and/or after the email subject or replace it completely with your own text. When you add the 'Insert subject text' action, you can type your own text into the text field. If you want to include the current subject text into the new subject, include this text token:
<Subject>

eg. If you receive an email with the subject "New product" and want to insert "Great " before the subject, then you can type this text in the action:
Great <Subject>
This will change the subject to "Great New product"