Technically he or she cannot. The President may veto any bill passed by Congress before it becomes law. Congress can override that veto with a 2/3 majority in both chambers.
Once it is law, the President is charged with seeing the laws executed.
Our founders specifically tried to establish an executive that was subordinate to Congress.
In practice, Presidents are pretty good at dragging their feet etc. to not execute a law they disagree with. This is extra constitutional, however, and not part of the basic framing.
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