The "Missing in Action" flow is one of the first flows we recommend you edit and activate right after you start using attendance.
It's very simple and easy to do.
Go to Message Center > Flows > View Flows
Click on "Missing in Action"
Then click each flow step and edit both the html and text versions.
After that make sure to activate the flow step for each one you edit.
Lastly, make sure to activate the flow itself.
Here's a quick video: