How can I replace a string in a file?

call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\Tools\VsDevCmd.bat"
cd C:\AutomationTestSamples\AutomationTestSamples\bin\Debug
start cmd /k vstest.console.exe AutomationTestSamples.dll /Logger:trx;LogFileName=%1/UnitTest1.trx /testcasefilter:FullyQualifiedName~AutomationTestSamples.UnitTest1.trx

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string[] str = File.ReadAllLines(@"C:\Test\test.bat");
            for (int i = 0; i < str.Length; i++)
            {
                if (str[i].Contains(trxName))
                {
                    str[i].Replace(str[i].Substring(str[i].IndexOf("/testcasefilter")), "/test");                    
                }
                
            }

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