Hi tayfun,
If you want to to add certain code then you can add from here. View below screenshot.
http://awesomescreenshot.com/06a5o5ja24
Hope this may help you.
If you like to change the excerpt value then the file is inside theme folder -> inc -> functions -> functions.php on line no 148, you can change the value 50 to your wish.
But before making changes in the code first make a child theme so that updating to new version. your customized code will not be lost.
Thank you!
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