1. The original law passed by Congress.
2. The executive orders and HHS regulations that were imposed since the original law, which exercise authority that is given to the executive branch under the original law.
Trump cannot overturn the original law on his own, but he can go a long way toward "fixing" the problem by rescinding many of the things that were imposed under Item #2.
Trump can also immediately cease and desist in any enforcement activity or court proceedings involving compliance with ObamaCare provisions that were left solely to the discretion of the Secretary of Health and Human Services, for example.
Yes and publicly direct DOJ to drop the heinous lawsuit against the little sisters of the poor (which Jeff Sessions will do regardless)