first you need to go to: Start>then type in the search bar>%appdata%.
it will bring you up to a screen, the folder .minecraft should be at the top.
open it, open the bin folder and stay dont do anything yet.

now, you need to go to this link ( [ Register or Signin to view external links. ] ) and save the file to your desktop.

next drag the file you just downloaded into the bin folder for minecraft and delete the file called minecraft.jar which was in there at the beginning, then rename the minecraft-1.8.1.jar to minecraft.jar and close the files.

now open minecraft and you should have minecraft beta 1.8.1