This article has been updated.
http://thehill.com/homenews/senate/376831-cochran-to-retire-from-senate-next-month
From the article:
“Under Mississippi election law, a special election will be held in November for the remainder of Cochran’s term, which runs through 2020. If no one receives 50 percent of the vote, the top two candidates would then compete in a run off.
Until then, GOP Gov. Phil Bryant can fill a temporary replacement, meaning the seat will stay in Republican hands for now.”