U.S. Constitution, Twenty-Fifth Amendment, Section 1
In case of the removal of the President from office or of his death or resignation, the Vice President shall become President.
Notes
Requires the Vice President to become President in case of the removal of the President from office, or of the President's death or resignation. In each of those three situations the Vice President becomes President, rather than only acting in the office for a time.
Use as reference
Lexinter.net | U.S. Const. amend. XXV, § 1 | https://laws.lexinter.net/constitution/us/amendment-25/section-1