The "Missing in Action" flow is one of the first flows we recommend you edit and activate right after you start using attendance.
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It's very simple and easy to do.
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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.
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After that make sure to activate the flow step for each one you edit.
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Lastly, make sure to activate the flow itself.
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Here's a quick video: