In filling out a form I checked a box by mistake. How can I remove the X in that box?

Just click it again... If it's not a real check-box but a radio-button, then you will need to clear the form, assuming you're using the free Reader. With Acrobat you could reset just that one field.

Hi Ernieo,

The form that you are referring above, did you save the form after purring the check mark there?

You should always save/keep a copy before Reader-enabling a file.

You may check- How to edit fields in PDF form after saving

also- How do I edit text after saving as a reader extended PDF form with 'additional features' enabled? (PDF Forms)  and see if that helps.

Regards,

Akanchha