This is the gold standard. Originally developed in the late 2000s and updated sporadically, version 1.5 or 2.0 is the most feature-rich.
These contain only user-created data (no copyrighted game assets) and are typically legal. pes 6 option file editor
Because PES 6 (Pro Evolution Soccer 6) is an older game, most editing is not done with a single "official" tool, but rather with community-created standalone editors. This is the gold standard