Despite being a much-requested feature, Zapier has still not given an indication on if, or when, this might be added.
For now, if you want to re-order the steps in a zap, you would need to delete the step, re-add it in the desired place and re-map all the fields. If any downstream steps in your zap map fields from the now-deleted old step, those will need remapping as well.
If you'd like to add your vote to this feature request and hopefully push Zapier to build it, you should contact their support. You can open a ticket here.
If you're finding it frustrating designing automation flows, but not being able to re-arrange steps, you might want to check out Integromat. It's a very similar automation tool, with a different style of UI to Zapier - and one that supports duplicating and re-arranging steps.