Official source page 54

(a) when his successor in office takes the oath of that office; (b) he dies whilst holding such office; or (c) the date when his resignation from office takes effect; or (d) he otherwise ceases to hold office in accordance with the provisions of this Constitution. (2) Subject to the provisions of subsection (1) of this section, the President shall vacate his office at the expiration of a period of four years commencing from the date, when - (a) in the case of a person first elected as President under this Constitution, he took the Oath of Allegiance and the oath of office; and (b) in any other case, the person last elected to that office under this Constitution took the Oath of Allegiance and oath of office or would, but for his death, have taken such Oaths. (3) If the Federation is at war in which the territory of Nigeria is physically involved and the President considers that it is not practicable to hold elections, the National Assembly may by resolution extend the period of four years mentioned in subsection (2) of this section from time to time; but no such extension shall exceed a period of six months at any one time. 136. (1) If a person duly elected as President dies before taking and subscribing the Oath of Allegiance and oath of office, or is for any reason whatsoever unable to be sworn in, the person elected with him as Vice-President shall be sworn in as President and he shall nominate a new Vice-President who shall be appointed by the President with the approval by a simple majority of the National Assembly at a joint sitting. (2) Where the persons duly elected as President and Vice President die or are unable for any reason whatsoever to assume office before the inauguration of the National Assembly, the Independent National Electoral Commission shall immediately conduct an election for a President and the Vice-President. 137. (1) A person shall not be qualified for election to the office of President if - (a) subject to the provisions of section 28 of this Constitution, he has voluntarily acquired the citizenship of a country other than Nigeria or, except in such cases as may be prescribed by the National Assembly, he has made a declaration of allegiance to such other country; or (b) he has been elected to such office at any two previous elections; or (c) under the law in any part of Nigeria, he is adjudged to be a lunatic or otherwise declared to be of unsound mind; or (d) he is under a sentence of death imposed by any competent court of law or tribunal in Nigeria or a sentence of imprisonment or fine for any offence involving dishonesty or fraud (by whatever name called) or for any other offence, imposed on him by any court or tribunal or substituted by a competent authority for any other sentence imposed on him by such a court or tribunal; or (e) within a period of less than ten years before the date of the election to the office of President he has been convicted and sentenced for an offence involving dishonesty or he has been found guilty of the contravention of the Code of Conduct; or (f) he is an undischarged bankrupt, having been adjudged or otherwise declared bankrupt under any law in force in Nigeria or any