I think your task can be done in a single loop. Are you familiiar with reading/writing files? If not, recommend spending a little time on the home page's FAQ section, VB6 & Earlier. The logic below is described in the Files faq.

Don't know how big the files are; you have 2 basic options, read entire file into memory & manipulate it, then output. Or use 2 files, open one for reading and the other for writing.

pseudo code looks a little like this, using 2 file method. Split() & Join() are VB commands.
Code:
   Open File for reading
   Open File for writing
   Begin loop
   Read line of input
       Does it begin with N?
       If not keep reading until it is found or end of file occurs
       once found ...
           Replace N with new N
           ' to replace a token in the line....
                Split() line on delimiter -- appears to be spaces
                Change value as needed
                Join() split line using same delimiter
           Write modified line to file
           Increment N
    Continue Loop
    Close both files 
    Delete old file, rename new file if needed
    done