Solved How to restore windows 8.1 (pre-installed)after formatting

I've already searched around but i could not find any solution, anyway here is the problem:
I had an envy notebook from HP with win8.1 pre-installed, then one year ago i did upgrade to win10 and, exept for some bugs everithing was fine.
Then i formatted in order to install only ubuntu 16.04 LTS 1 month ago but now i would go back to win8.1.
Is it possible to do? How can i do it?
I've also tried to install win7 with an old CD but at the start up it says: "press any key to begin installation" but after pressing it just run Ubuntu normally.
Thank for your help.

Do you want 8, or10 back on you can download win 10 iso and put that back on you will need to delete all the partitions on the drive first as they will be all Linux

samuria said:

Do you want 8, or10 back on you can download win 10 iso and put that back on you will need to delete all the partitions on the drive first as they will be all Linux

Click to expand...

I want to have 8.1 back
So what you are saying is to download it from somewhere (before i've seen some guide about this) then delete Ubuntu (how?) and then boot win8.1?
And what if win 8.1 doesn't boot like win 7?

You boot win8 and during install when it ask were to install it shows you the disk and lets you delete all parition or you could boot linux using a live dvd and delete all partition first. Doesnt the pc have a rescue partition?