Editing/Saving a row in a CSV file

After following this topic I am able to create the new row but my question is how do I save or write the new line to the file?

I tried 'StreamWriter' but it only writes the newly created line.

Any suggestions please?

Here is my code so far:

string path = @"C:/CSV.txt";

string[] lines = File.ReadAllLines(path);

var splitlines = lines.Select(l => l.Split(','));

foreach (var line in splitlines)
{
   if(line[1].Contains("34"))
   {                                        
     line[1] = "100";
     var newline = string.Join(",", line);
     StreamWriter sr = new StreamWriter(path);    
     sr.WriteLine(newline);
     sr.Close();                                      
   }
 }