You can erase parchment by scraping away at it with a sharp metal tool or a pumice stone. If that sounds difficult, it is. To do this reasonably well, you should consider getting a professional - a "parchment restorer". However, those are likely in short supply these days. As you can see in this palimpsest below, you will have to make do with an incomplete erasure. It will makes sense to construct your "fresh" book from from the parchment leaves that you erase the most effectively or are the least damaged. Recompile those leaves into a manuscript and you're ready to go.