You can add these lines to the interior of your functions.php. I suggest you create a child theme for wordpress before adding the code. The themes are updated and you will lose your changes otherwise.
Enter this code at the end of the file, remember that in this example the number 20 represents the maximum number of words that can be included within the summary.
To achieve an optimal result, you should do some testing. Normally 20 words for a summary, are a few. If you have any problem with my guide or you is not clear, leave a comment and I'll answer as soon as possible.