1. If you don't already have a custom template setup, you need to follow these steps to create one since your customized Engagement Letter will need to be a part of the custom template: https://proconnect.intuit.com/community/proconnect-tax-discussions/discussion/re-intuit-link-wants-t...
2. Stay on the same screen as #1, click on the name of your custom template.  On the line for Engagement Letter, click on Edit;
3. Make whatever changes you'd like to the text and hit Save.

Please refer to the thread in the hyperlink above for how to send requests with the custom template(s) you have saved.