Here are some text-replacement examples.
Try applying each substitution to the example.

1. :s/his/her/g

This is his idea, or so his imaginary friend said.

2. :%s/\(good\|nice\)/awesome/g
Linux is good. Life is nice.
Linux is good. Life is nice.

3. :%s'<,'>s/1/2

Let's just replace 1 with 2 in the following two lines
some numbers 1 3 2 1
more numbers 1 1 4 4
don't affect these numbers 1 2 3 4.