How can I update a checkbox from a plugin?
If I understand correctly you're talking about a Checkboxes field type. Because you can have multiple checkboxes within such a field, Craft stores an array to the database.

Try if this works:

$entry-&gt;setContentFromPost(array(
    'agreeToTerms' =&gt; array(
        craft()-&gt;request-&gt;getPost('fields.agreeToTerms'),
    )
));