In the dashboard, click on the Templates icon:
Locate the template you want to edit in the list.
(Tip: if you don't see the template you're looking for, make sure you are looking in the correct Team. Change the Team you are viewing in the drop down menu at upper left).
Click on the three dots (...) to the right of where the template you want to edit appears in the list and select "Edit."
Clicking "Edit" opens the template editor screen, where you can make changes to your template. For detailed instructions on using the template editor, start here.