I assume that the pillars are supporting some kind of roof structure and therefore hold weight. Providing the joists are sound underneath, I would suggest removing a section of boards at a time starting at the top of the steps with a suitable threshold board. Depending on the distance between each pillar, use a 2x4 with a brace at the top as a "Preacher" to support the roof as close to the pillar as possible allowing enough access to work. Remove about 2ft at a time so that you can still work on the existing boards. Remove the screws to the railings as you go and replace with galvanized or stainless as you go. With a project like this safety is a concern, so only remove a few boards at a time. Material should be 5/4" pressure treated or composite boarding, again using galvanized or stainless steel screws.
If you have extensive damage to the structure other than what you can see.........re post this and we'll come up with a new plan.
Last thing.....give your neighbours the heads up when your working up there,
"I'm not as green as I am cabbage looking"