If the Supreme Court determines that the election of the President-elect to be invalid, a fresh election shall be held within sixty days after the determination. -- Article 140(3)
This is the final clause of Article 140 that deals with "questions as to the validity of a presidential election". It is the clause under which Raila Odinga asked the Supreme Court to invalidate the election of Uhuru Kenyatta. It is not akin to any of the clauses of Article 138 that deal with runoff elections, such as when no presidential candidate garners the 50% + 1 votes needed to win and a fresh election has to be conducted between the top two candidates (clause 5) or among the candidates that garnered the most votes (clause 6).
An alumnus of the Twitter School of Law is peddling the extremely erroneous notion that the election that the Supreme Court directed should be held in sixty days will be between Uhuru Kenyatta and Raila Odinga. Of course, like all legal simpletons, his opinion has as much weight as a castle in the air or a pie in the sky. Even without the benefit of the detailed judgment of the Supreme Court, it is quite clear that the Supreme Court did not say that a runoff be held under Article 138(5) or (6), but that the election of Uhuru Kenyatta as president-elect was "invalid, null and void" and, therefore, a fresh election will be held. This means, for the illiterate, all the presidential candidates on the ballot on the 8th August shall be on the ballot at the next presidential election. (There doesn't seem to be a provision for what to do when a candidate wishes not to participate in the fresh election.)