ISaveGame saveGame = SaveGameFactory.LoadFile("my_game.ck3");

// Inspect data
string id = saveGame.State["currently_played_characters"].First().Content.Text;
INode player = saveGame.State["living"][id];
string name = player["first_name"].Value.Text;
decimal? stress = player.GetDescendantOrNull("alive_data", "stress")?.Value.AsDecimal;
Console.WriteLine($"{name} has {stress ?? 0} stress");

// Modify data
if (stress.HasValue)
    player["alive_data"]["stress"].Value.AsDecimal -= 25;
saveGame.DisableIronman().WriteFile("my_game_modified.ck3");