You don't. Nick Baumann explains:

Could the pope be impeached?
No. There's no procedure in church law that allows for a pope to be impeached. It's an open-and-shut case, Watson.
The pope has what is probably the world's safest job. It's essentially impossible to dislodge him without his consent. Even if he's sick or otherwise incapacitated, he's still the pope until he says otherwise. Theoretically, a pope can resign—and some of Benedict's critics have urged him to do so. But it's unlikely that this will happen. Canon law requires that any resignation be "freely made and properly manifested." The last pope to step down from the post was Gregory XII, in 1415.