Replacing fascias and soffits is pretty straight forward, the job is done in two ways, either you replace the whole wooden section, which you would buy full replacement fascia boards, or you would cover over the existing wood with upvc boards that are called pvc capping boards, these are around 10mm thick as against the full replacement boards which are around 18mm thickness.
If you go over the top with pvc capping boards make sure that you decorate or at least treat the wood thoroughly first. to use full replacement fascia boards you would need to strip out all the wooden parts first and replace with uPVC roofline fascias and barge boards.