Howdy Ilkka,
I would love to help you with this.
Actually, you can not override QR.php. Only files inside of the /views/ folder can be overridden. There is good reason for this. Almost any file outside of that folder is extremely to break compatibility with plugin updates, thus if you overrode it you would likely find yourself unable to update without getting errors.
However, I bet we can still easily accomplish what you want to do. I’m guessing you are trying to alter the HTML code output by the inject_qr function. You can still do this, and stay compatible with plugin updates! You will want to stop our plugin from outputting that HTML, then copy/paste the code that powers it into one of your own files such as the functions.php in your theme.
Here is what I would do if I were you:
https://gist.github.com/elimn/8a0a0dfd02842c30ff3d27e895819837
Does that all make sense?
Cheers!
– Brook