need to change lost ignition key

It's generally easier to have a dealer or a locksmith make you a new key than it is to replace the ignition, if I'm reading your question and title correctly.

If you really want to replace the ignition, that still won't help with the doors or the trunk so you'd have to buy a complete lock set. Definitely not worth the effort.

If, however, that's really what you want to do, we'll need more information. Peugeot has made a lot of cars over the years, and there's no way we can help without knowing the year and model of yours. You'll need to update your question and/or add a comment, or at some point we'll just have to archive your question as unanswerable.