Prerequisites:1.Download windows 10 from here.
2.Download and install power iso.
How to install windows 10 on windows 8 or 8.1:There are Three ways you can install windows 10 on previous versions of windows.
Thing you need to do before proceeding further.
1.Create a bootable windows 10
2.Create a bootable Dvd.
Must read: how to create a bootalbe windows 10 usb drive.
Method 1: From boot menu1.Attach the bootable pendrive to usb port.
2.Get into boot menu and set first priority of boot to usb
3. When you boot normally after attaching the pendrive.It boot to pendrive first.
4.You can continue installation after that.
Method 2:From usd or DVD
1.Put the dvd/pendrive
2.Open it.you will a setup file
3.Click on setup file and wait for few minutes for it to load
4.you will granted with options
a.Keep all your setting and personal data
b.Keep only your personal data
c.Erase all your settungs and personal data
5.select the option carefully and procede the installtion
Method 3:From the extracted files on hard disk
1.Extract the windows iso image on hard disk
2.make sure that the extracte files are in a single folder
3.click on setp application and launch it
4.Preceed through installtion carefully