Ok. I see that. Your code isn't renaming the files because you aren't doing anything that would rename the files.

Each time round your loop, the $zip variable gets the name of one of your files. You then perform a substitution on that variable so that '.log' is changed to '.txt'. But all you've done is to change the value in the variable. That variable doesn't have any connection back to the file that's sitting on your hard disk. In order to rename the file, you need to call a function that asks the operating system to rename the file. And that function (as you've been told) is called rename.

Is that clearer?