Editing a message template causes an important question to be asked "Do you want to also update future scheduled messages that use this template? Yes or No."
FROM THE WEB APP
Step 1: Go to Settings > Message Templates > Edit.
Step 2: Select the message template you want to edit.
Step 3: Check the "...update all existing messages..." box at the bottom.
Step 4: Save.
FROM THE MOBILE APP
Message templates can only be edited from the web app at this time. You can use the mobile app to send messages using templates, but editing the templates is only possible from the web app.