Franklin Roosevelt served three terms, was elected for a fourth, but died early in that term.
presume it would require an amendment to the constitution, though - the rule about serving more than 2 terms appears to have been an informal convention before the
22nd amendment was passed in 1947.
i'm not sure what has to happen to pass an amendment to the constitution, but presume it would have to be set off by the incumbent administration and get through both houses.
for what it's worth, the wikipedia article says it's fuzzy about whether someone who's already served 2 terms as president could get elected vice president then take over as president if the president then died or became incapable of doing the job...