Uncheck a check box

I have a student that is creating an app with a "to do" list using check boxes. He wants to be able to uncheck the box after the task is complete. 

How does he do this?

I'm not sure, if I understand the problem...

a checkbox can be checked or unchecked

so if you want to uncheck a checkbox, just uncheck it

or is the question, how to save the state of the checkboxes for the next run of the app?

if yes, the answer is: use TinyDB...

set Checkbox1.checked  false

The student may also need to put some actions in the Changed event handler depending on his/her actual code.