尝试一下:

var path = @"c:\temp\test.txt";
var originalLines = File.ReadAllLines(path);

var updatedLines = new List<string>();
foreach (var line in originalLines)
{
    string[] infos = line.Split(',');
    if (infos[0] == "user2")
    {
        // update value
        infos[1] = (int.Parse(infos[1]) + 1).ToString();
    }

    updatedLines.Add(string.Join(",", infos));
}

File.WriteAllLines(path, updatedLines);