1. make sure you have the excerpt element toggled on in your view settings.
a. click screen options.
b. click the Excerpt item to toggle it on.
You will now have the Excerpt block visible and you can add your Excerpt in here.
2. Once you add your excerpt, this will display on the front end on your site. This will replace the text under the blog title before the main content.
NOTE: If you don't fill in the excerpt, the system will automatically use the first few lines from your article. The excerpt can only have up to 55 words, so it's best to keep it concise and appealing.