The most widely-anticipated policy change is moving the U.S. embassy in Israel from Tel Aviv to Jerusalem (which in the short-term would require just switching the signs in front of U.S. diplomatic facilities). Another possibility is kicking the official representatives of the Palestinians out of Washington. A third is ending aid to the Palestinian Authority (PA).
Why focus on these three? Among other reasons, because all are already required by law: a 1995 law requires the transfer of the embassy; a 1988 law bars the Palestine Liberation Organization (PLO), the recognized representative of the Palestinians, from operating in the U.S.; and Congress has for nearly two decades passed legislation every year barring funding to the PA. However, Congress also gives presidents the authority to waive implementation of each of these laws, in an arrangement that lets legislators score political points with pro-Israel hardliners, while leaving space for presidents to preserve responsible policies.
Until now, presidents from both parties have consistently exercised these waivers; by merely ceasing to do so, Trump could change any or all of these policies, immediately and on his own. (Lara Friedman, Think Trump’s Policies On Israel-Palestine Have Nothing To Do With You? Think Again, Huffington Post, January 6, 2017.)Furthermore it turns out there is a law which requires the United States to withdraw and defund any United Nations organization which admits the Palestinians as a member. That is one reason why UNESCO passed their recent resolution about Jerusalem. America was not a member and paid nothing to them and so had no say because of this law.