Here's a way to modify a file in place:

First, make a copy of your file and put it on your desktop. In this example, I called it apache.txt.
Second, open a terminal.
Third, from the cmdline, enter:

perl -pi.orig -e 's/\bis\b/are/gi' /user/Desktop/apache.txt
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It'll modify the original file and create a new apache.txt. It'll save the original as apache.txt.orig.