As anonymous Monk has pointed out, these kinds of changes in a file can be handled by Perl one liners:

~/perl/change$ cat change.txt
Hello I am Kelly. Hello I am David. Hello I am Mark.
~/perl/change$ perl -pe 's/David/Josh/i' change.txt
Hello I am Kelly. Hello I am Josh. Hello I am Mark.
~/perl/change$ perl -p -i -e 's/David/Josh/i' change.txt
~/perl/change$ cat change.txt
Hello I am Kelly. Hello I am Josh. Hello I am Mark.
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