The easiest and cheapest way is to just go over top of them with vinyl (cheapest) or aluminum (a little more money, but better quality). With this stuff (it is thin), you just go right over top of the old stuff. If you want quality though, that will last, then pull the old stuff off and replace it with wood. Whichever you do,your local building supply store can make recommendations from what they carry. Be sure to repair/replace any rotted framing pieces that are behind the soffit/fascia before covering it back up. Don't neglect to get a good coat of paint on any wood you use.