You launch the Email Template Editing Tool from the Administration/Maintain Email Templates menu item in both Atrium 8 and Atrium 10.

Once you have selected the template you want to edit:

Click the Edit Email Template button in Atrium 8:
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In Atrium 10, you click the Edit icon on the right of the selected template:

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You will then go to the same editing tool from either version:

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Note that 'Placeholders' are words in upper case with underscores between words.  For example COMPANY_EMAIL is a placeholder that will substitute the email address of your company.  CUSTOMER_EMAIL will substitute the email address of your client/customer.  See this link for a list of definitions of the placeholders:

Any text you enter that is not a placeholder will simply render as that text in your email message.  

Note that you have tools for adjust size and color of the font and also to insert links and images.

Once you have finished with your editing, click the 'Save Email Template' button in the lower right corner before closing the tab in your browser.

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The next time this email template is triggered in Atrium, it will contain your changes.