First you need to download fresh and official windows ISO filehttps://www.microsoft.com/software-download/windows10
Install or upgrade Windows 10 PRO and enter key
If your existing OS is Win 10 Pro, Simply enter the key for activation.
#For clean install
1.Go to above link
2.Click Download tool now.
3.Create installation media for another PC(need 8gb USB)
4.Set your PC to boot from USB (Change bios setting)
5.Follow next direction(Install win 10 PRO)
– If your existing OS is Win 10 HOME, Simply enter key for upgrade.
Upgrade will be started automatically.
(If it goes to Enterprise, Re-enter key)
– Alternative way to upgrade from Home to Pro
1. Disconnect from the internet
2. Change key to VK7JG-NPHTM-C97JM-9MPGT-3V66T (windows 10 pro default key)
this will upgrade to Win 10 Pro, without activating
3. After upgrade, Connect internet and Enter Win 10 Pro key
– If your existing OS is Win 7 (8 or 8.1)
1. Go to above link
2. Click the ‘Download tool now’ button
3. Choose ‘Upgrade this PC now’
4. After upgrade to PRO, Enter Win 10 Pro key
(It might upgrade to Home. In that case, Try upgrade again from Home to Pro)This content is locked
you can download iso files from here also