Does anybody know where i can find it to make a backup ?
TY

Sorry for the late reply, you probably already figured out where it was. It's the DAT file named "system" (without the ") located in this directory: C:\GOG Games\Astebreed\data

You should also backup "RankingStageNormalXb", "RankingTotalNormalXb" (both contain your leaderboard scores) and "config.json" (configuration file) all located in the same directory as the savefile.