To edit a template, enter your templates from the + button in the navigation menu.
In this page you will find both My templates, which are your organization's templates and Public templates.
Select any of your templates from My templates and then next to Create parallel select the "More options" button with 3 dots.
Choose Edit template and start modifying your template. The changes will be saved automatically.
Public templates cannot be edited, but you can always copy the template using Save to edit and then follow the steps above for your newly copied template.