impossible to save modify in file

Instead of editing core file, just use this filter to change the thumbnail size.

sorry, but where can I use a filter?
Now I try to change wp-store-locator/frontend/class-frontend.php

It’s wrong?

The filter code goes into the functions.php inside your active theme folder.

Changing core files is a bad idea because you will lose your modifications when the next update is released.

thank you very much