Deleting a custom 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: Click the "Trash" icon at the right of the template.
Step 4: Click "Yes" to confirm.
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.