It's not as trivial as you may think: although in your example it's fairly simple, it gets a lot more complicated very quickly - because text files have no concept of "words" just characters. Which means that unless - as in your example - the two words to be exchanged are the same length you need to copy the file up to the beginning of a word, copy it's replacement in, skip the found word and then continue looking, copying while you go.
When complete, delete the input file and rename the output file.
The way I'd start is by reading the input and isolating the words, comparing and copying to the output file as you go. Compare each word to output before you write it and replace as necessary.
That's fairly easy to do: start by locating each word and just copying them to the output (complete with non-word characters) so you can easily compare the input and output files to make sure it works; then add the compare-and-swap logic.
We are more than willing to help those that are stuck: but that doesn't mean that we are here to do it all for you! We can't do all the work, you are either getting paid for this, or it's part of your grades and it wouldn't be at all fair for us to do it all for you.
So we need you to do the work, and we will help you when you get stuck. That doesn't mean we will give you a step by step solution you can hand in!
Start by explaining where you are at the moment, and what the next step in the process is. Then tell us what you have tried to get that next step working, and what happened when you did.
If you are having problems getting started at all, then this may help: How to Write Code to Solve a Problem, A Beginner's Guide