Overwrite vendor classes
In composer.json, under [„autoload”][„psr-4”], add an entry with namespace as the key and path as the value:
Copy files you want to override under that path (keeping sub-namespace directory structure) and edit them there. They will be picked in preference to the library package’s original „classpath”. It would seem that namespace->path mappings added to composer.json in this manner are considered before those added by required packages. Note: I just tried it and it worked, though I don’t know if it is an intended feature or what possible gotchas are.