Out of the box the current default theme after installing WordPress shows all of your posts in the long format.

To change the theme to just show a short summary as per the screenshot below read on….

WordPress post excerpts view


Go to the Dashboard -> Appearance -> Editor

WordPress - appearance - editor


Now edit the file called content.php by selecting it from selection on the right


Search for the term ‘the_content’ using ‘Ctrl + F’ on a Windows PC or ‘command + F’  on a Mac


Now replace the word ‘content’ with ‘excerpt’ and click on update file.

change to excerpt

All done, now post pages with have the excerpt.