Should I buy the premium version to change the flags?

Once the custom flags are uploaded, go to Languages > Settings. Click on the “URL modifications” settings and then on Save Changes.

Note: your custom flags won’t be used on admin side.

Source:
https://polylang.pro/doc/can-i-use-my-own-flags-for-the-language-switcher/

Thank you mononox for your reply but “URL modifications” settings are only in premium plugin ?

Oh, I wasn’t aware of that. Sorry about that.

You should be able to put new image files in using CSS only though. Just look at the source code to find the classes and override the statements using !important.

ok, I’ll try that
Thank you !