First of all using a while(true) loop is not a good idea. It would be better if you did it like this so it is easier to read :
String textinLine=br.readLine();
while(textinLine != null)
{
sb.append(textinLine + “\n”); // if you don’t put the \n you will have all the code on one line as fer said
textinLine=br.readLine(); // do not put the \n here unless you want to experience an exquisite exception.
}

Then you should create a method to do replacements easier without needing to copy/paste the code each time, and besides, it is actually doing only one replacement and this is not kinda effective for people wishing to make multiple replacements. What do you think about this :

// if you want to put this in the same class as your main do not forget to use the static keyword to declare the method
public void replace(String textToEdit, String textToReplaceWith) {
int cnt = sb.indexOf(textToEdit);
while(cnt != -1) { // so it will replace every occurence until there is no more left
sb.replace(cnt2,cnt+textToEdit.length(), textToReplaceWith);
cnt = sb.indexOf(textToEdit);
}
}

To continue these three lines of code are an error of logic since fs is used to create in which is used to create br :
fs.close();
in.close();
br.close();
They should be reversed :
br.close();
in.close();
fs.close();

Have a nice day.

after I replace the word of “fern”
it will make a one line ” name: NEWNAMEage:20 ”
which is I want to keep the original position and just replace the word I want to replace.
please mail me how to do it. Tnx!