Tell the story of your switch
A career changer's resume can look confusing on its own. The cover letter is where you explain it: what you did before, what drew you to the new field and what you've done to get ready. Keep it short and forward-looking - the reader cares more about where you're going than why you left.
The strongest stories show the change starting in your old job: automating a report, training colleagues or solving a problem with the skills you now want to use full time.