Well, basicaly you have 2 ways to do that. If it's just a matter of editing one file once in a while, I'd use vi for that. If you want to do that on a regular basis, a shell script would be better suited.
vi method :
vi pool.txt
(position cursor on the 1st line)
type : 5dd (the first 5 lines will disappear, don't panic)
type : Shift-G (cursor will jump to the last line)
type : p (5 lines will magically reappear at the end of text)
type : 5 and (Up Arrow) (cursor will jump up 5 lines)
type : 5dd (those 5 lines will vanish)
type : :0 (the colon sign and the number 0), that will position you at the top of the file
type : Shift-P (5 lines reappear on top of the file)
type : :w pool2.txt (colon sign and the letter w + name of the new file)
type : :q! (colon sign, the letter q and exclamation mark) to quit without saving the changes to the original file
This seems rather obnoxious, but in fact, it take just about 3 seconds to actually do that
Actually, nobody would want to write a script to do that, vi is quick enough. But if you really want to do it without using vi then :
$ tail -5 pool.txt > pool2.txt ; sed -n '6,15p' pool.txt >>pool2.txt ; head -5 pool.txt >>pool2.txt
... will do the trick nicely