Hi,
I have been developing in C# for 6+ Years. Not only can I help you with this query, I would like to offer you my 'On-hand' help and assist you with further queries. I'm sure we can make an arrangement.
As for this issue, there are multiple ways you can achieve this. You can replace the 'First found instance' of the string and create a dictionary or list or queue of the words you want replaced, then loop through and change each instance of the string. As you change it, the next value will be the next to change in the loop. You can also use Regex to set the amount of strings you replace, you could extend this to give 'random' strings to 'random' found strings as well.