If you want to change specific career mechanics rather than just player stats, these tools are helpful: PES 2017 ML/BAL Starting Year Changer
If you prefer not to use third-party software, you can use the game's built-in : bal pes 2017 editor free
: Often used alongside specific PES 2017 tables to directly modify player attributes like skills and overall ratings (e.g., boosting a player to a 99 overall ). If you want to change specific career mechanics