To edit the subject line for an estimate being sent via email, you can do this while sending the estimate or from a previously saved estimate.
When sending a recently generated estimate, you can make changes on the Share Estimate screen or directly in the email subject line before sending.
If you have a saved estimate that you would like to edit, you’ll want to tap into the saved estimate, tap the three dots on the upper right > send estimate > preview(x2) > share estimate > email > select email from your phone > edit subject line.