By default, excerpt is automatically taken from the first few words of your post. If you’d like to have control over the excerpt length and words, you can choose the manual way by writing a custom excerpt for each post following these steps:
1. The Excerpt field is hidden by default if it has not been used before. Therefore, click the Screen Options tab at the top right when writing or editing a post
2. Make sure that Excerpt is checked.
2. Under the post editor, you should be able to see Excerpt field, so you can type in excerpt for the post.