VS 2010 VB.NET: appending a text file of url's (at specific lines)

Ok lets say I have a text file called "google.txt", which contains 500 google links (going to different keywords or new stories etc.)
What i want to do...is at line #50, #75, #100, #150...simply replace the existing "line" or google url in this list with http://finance.yahoo.com

I know how to insert a string to a specific line # in a listbox (on the vb form) using the following code...
Listbox1.Items.Insert(3, "http://finance.yahoo.com")
^ where 3 is the listbox line # i want to insert my text

Can someone please show me how to do a similar thing to an "already existing" list in a text file?

Thanks!!!
Jenn