You say you want more details, so I hope engine's explanation helps, but essentially it is as I said...select what you want to delete, delete it, and insert your new content.

You can use F8, OR you can use the mouse to select the content you want to delete (and delete it).

You insert new content by using Insert or a CopyPaste.

Other than that, you may have to GIVE more details.

As engine states you may need to clean up styles.