Overriding extension files is similar to overriding theme files:

1. Find the file you want to modify, where can I find it?
2. Create an extension in your theme's src folder if you haven't already, with the same name as the extension, files of which you're modifying. How to create an extension?
3. Create a new file in your extension, which:
   1. Has the same name as the original file
   2. Is located in the same folder (relative to src) as the original file, but under the newly created extension-specific folder**.** The pattern for the folder to create an extension is src/<EXTENSION PACKAGE NAME>/<PATH AS THE ORIGINAL EXTENSION>.
4. Modify the files according to your needs.