In SED the substitute is represented by an s. It can be used to replace or substitute characters. Go to your terminal and type in

echo N

This will of course print N

Now for using Sed
Type in

See? Very simple. Now for a bit of explanation

echo sets the variable to out put and | gives it to sed to manipulate

you execute sed by(sed) then the argument for substitution 's' then the /<String to replace>/<What to replace it with/

Try toying around with this, it is very powerful.