How to make categories have thumbnail preview?

https://codex.www.ads-software.com/Category_Templates

Thanks but where do I edit category.php

Admin/Appearance/Editor. Or download the file to your machine using FTP, make your changes in a plain text editor and upload the new file back up to your theme folder.

Sorry, what I meant was I cannot find the file category.php in my theme. Where exactly is it found

Perhaps your theme doesn’t have a category template file. Please see the link above for details of what file is used in the absence of catgeory.php.