Replacing Content Via Plugin – Question

I am writing a plugin that searches posts, finds a particular sting, parses out the data, then saves parts of it to a table. Everything is work great. Now I need a little help. After I have done the above I want to basically rewrite the post with new content. Do dump what was there and replace with a new massage. I don’t want to filter it so it is processed every time. I want to replace it in the database. I already have it getting the post id correction. Can someone point me in the right direction for replacing content?

Thanks.