Depending on how your tempalte is set up will depend on how this works. If it is set up as per the WP core functionality then you just need to click on the break symbol where you want the extract to stop. The break symbol is the two boxes intercepted by a dotted line (next to the spell check in this image)
What I mean is...When you click on any of the blog posts, you get to a page which shows the whole blog post. How can you edit how these pages are displayed?
The issue comes about because I have used a Sidebar file (Sidebar.php) that Wordpress uses to pull in a small description about the blog which sits at the top right of the main blog page. When you click on any of the blog posts, the layout of this page takes up much more space and pushes this Sidebar content down. I would prefer to remove the Sidebar content from these blog pages but keep it on the main page. I just can't seem to edit the individual post pages.