Editing a custom message template is easy!
FROM THE WEB APP
Step 1: Go to Settings > Custom Templates.
Step 2: Select the message template you want to edit.
Step 3: (Optional) Check the "...update all existing messages..." box at the bottom.
Step 4: Save.
FROM THE MOBILE APP
You can use the mobile app to send messages using templates, but the message templates can only be edited from the web app at this time.