How can a president override a congress bill?

He cannot. The president can veto the bill, returning it to Congress without his signature. Congress can override the President's veto by a two-thirds vote of both the House and the Senate. If the Congress overrides the President's veto, it becomes law even if he doesn't like it.

If the President refuses to enforce the law, he is violating his Constitutional oath to ensure that the laws will be faithfully executed, which would be an impeachable offense. Congress could then "fire" the President by removing him from office.