When using WordPress 5 or higher, you can set the excerpt by using the "Excerpt" tab shown on the right side of the edior:

To set an excerpt whilst using the Classic Editor:

1. Go to the Posts section of your dashboard and select the post you would like to edit.

2. At the top right of the screen, click the Screen Options tab, as shown below:

3. In the options that appear across the top, make sure the "Excerpt" box is enabled, as shown below:

4. This will add an extra option to the bottom of the screen. If you scroll down, you should see a new "Excerpt" field. You can use this to add any text you like, and this will become the post's excerpt.