That would require the use of SED. A tutorial can be found here
To replace i's with another letter try
echo "This is a Test" | sed -e 's/i/x/g'
That will replace all the i's with x's.
To replace is with hello try the following:
echo "This is a Test" | sed -e 's/is/hello/g'
I apologize for the crude explanation. I would give the actual output w/ examples but I'm not on a linux machine currently so I cannot do that. With the code samples and a tutorial you should be good to go. Hope this helps!