Sometimes this happens when mistakenly you forget to mark that you can be claimed as a dependent by someone else when completing your return. If you have already filed your return claiming your exemption, the only way to change that is to amend your return, removing your exemption. In order to have the amendment processed, the amended return will need to be completed, printed, signed and mailed. If your guardian is entitled to your exemption and still wants to file you as a dependent on this years' return, his/her return will also need to be printed and mailed for processing.
See link below that explains how to amend return in TurboTax:
https://ttlc.intuit.com/replies/5114064
