how to replace a word in specific line in text file

thanks for answering

i want when i complete some task it goes to the specified line and change that faux to ok and continue for exemple i have 3 line

1 abc abc .... faux

2 abc abc ......faux

3 abc abc ...... faux

when complete the loop and all work return to the line in the text file with exact number and replace it with ok

like replace the faux in line 2 with ok

or line 3 with ok and so on

i hope this making things clear