i know somebody mentioned it before but it's not clear to me: how can i move from rawhide (f15) to f15 (and not to f16) and, how can i downgrade from rawhide (f16) to f15?

--
"The end is important in all things."
 -- Yamamoto Tsunetomo  / Hagakure / Ghost Dog: The Way of the Samurai
"The beginning is the most important part of the work."
  --  Plato