Assassination? Resignation? Traditionally the office just expired in six months. No additional act was needed. Sulla when he marched on Rome the second time had himself appointed dictator with an innovation: he had a specific brief without time constraints. That was "for the making of laws and the establishment of the Constitution." Even so, he resigned at or before the 6 month mark and for the rest of his rule used the consulship only. When Caesar first used the dictatorship he resigned after something like 10 days. He started to accept longer terms later on in his career, but even the supposed "dictator for life" was just really "dictator perpetuo;" which is something like "dictator for as long as necessary" and not necessarily for life, i.e. he was allowed to resign.

There wasn't something like American impeachment, if that's the sort of thing you're looking for.