You can follow along the steps in this doc to Edit an existing template. Changes made to a template won't alter your automations that use the template.

To create a template navigate to the Templates page and click on the three dots on the top right corner and click Edit on the template you want.
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In the email composer, edit your email template. Every element you add here will be available when you select the template in your newsletters and automations.

Once you're done creating your template, click the Save and Exit button at the top-right of the screen.

That's it you've edited a template with SmartrMail 🎉