Write Text From Textbox Control To Text File .txt At Specific Location?

Is it Possible for me to read text from textbox control and write it to .txt file at specific location.

for an instance.... say below with quote is my Text in .txt file:

"THE QUICK BROWN FOX JUMP OVER THE LAZY DOG"

and with programming code I want to change some text in the same sentance become....(see below)

"THE QUICK GREY FOX JUMP OVER THE LAZY CAT"

so you can see the word BROWN change to GREY and word DOG change to CAT