To change your dependency status on a tax return that has already been filed and accepted, you must prepare, print, sign and mail an amended return form 1040X. After starting the amended return, uncheck the box in My Info that says you can be claimed by someone else.
Here are instructions for amending a return: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
