How can I replace a string in a file?

Hi,
I have a batch file as below:

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

I want to read a batch file line by line and if the line contains "UnitTest1.trx",
I want to replace "/testcasefilter:FullyQualifiedName~AutomationTestSamples.UnitTest1" with "/test".

I tried the following code but when I opened the file nothing was replaced.  Any idea please?

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");                    
                }
                
            }