To change the email address that appears on your proposals footer, you'll want to go to LMN home. From the side menu click Account then Company Info. In the Company Info section it's the Email field. Once the Email field has been changed this will be reflected on the proposals that you print.
Please note: Each user can have their own unique email here. If you want the same email for each user account you'll need to change for each user following the steps above.