using System;

using System.Collections.Generic;

​

​

String[] file = 

{

    "700 104 1 0", 

    "16 Autodesk AutoCAD 19 ASM 221.0.0.1871 NT 24 Tue Aug 16 09:02:14 2016", 

    "1.005e3 9.9999999999999995e-007 1e-010"

};

​

int lineWithParam = 3;

​

String[] splittedLine = (file[lineWithParam-1]).Split(new char[] { ' ' }, 2);

​

if(splittedLine.Length == 2)

{

    float fact = 0.0f;

    String newFact = splittedLine[0];

    // or how ever you want to modify your factor

    if(float.TryParse(splittedLine[0], out fact))   

    {

        newFact = "" + (fact * 2.0f);

    }

    file[lineWithParam-1] = newFact + " " + splittedLine[1];

}

​

(new List<String>(file)).ForEach(x => Console.WriteLine(x));