Modifying 'the_contents' before saving?

Hi!

I am coding a custom post type.

Using the hook ‘save_post’ I am able to manipulate custom meta data before it’s saved to the database

Like this:
add_action('save_post', 'dosomething'), 10, 2);

I would also like to be able to manipulate the excerpt and the contents, but I don’t know how to set them?

In other words: we have a ‘get_the_contents function’, but there is no ‘set_the_contents’ function.

So how do I set the value of the content (and the excerpt)?