there is a text file containing several lines of text, how do i use the seek statement to obtain the writing position for a certain place corrosponding to the string character i want??

Eg.
///////
this is a test ok
second line

forth line, and the last line.
/////////

how do i get the above to appear like the below using the
seek and put statment??

///////
this is a test ok
second line

forth line, and the las[new string entry]t line.
/////////

thanks in advance