Windows Update released Windows 8.1 Update to all existing Windows 8.1 users as mandatory update. In this article, we will use a free tool called Rufus to create bootable USB then boot from the bootable media to begin the installation of Windows 8.1 Update. This update method also helpful when PC is unbootable or you like to install Windows 8.1 Update on newly assembled computer.
Step 1: Connect USB drive as bootable media to clean install Windows 8.1 Update.
* Make sure you backup all data we need to format the drive in order to make it bootable.
Step 2: Download Rufus on working PC and run it on PC running Windows XP or higher version of Windows with Administrator privilege.
Step 4: Once Rufus is running, under Device section, select USB drive.
Step 5: Next, select partition scheme as MBR partition scheme for BIOS or UEFI computers and select the File system as FAT32, as a bootable USB formatted with NTFS can’t be used to install Windows on a UEFI based computer. However, if you’re sure that your PC is not a UEFI PC, you can simply select NTFS as the File system. But if your PC is UEFI, be sure to select FAT32.
Step 6: Click the small CD/DVD icon next to ISO image button to browse to Windows 8.1 Update ISO image file and select it.
Step 7: Finally, click Start button and then click OK button when you see the confirmation box to begin preparing Windows 8.1 Update bootable USB.
Step 8: Once bootable USB is ready, restart PC, make necessary changes to BIOS/UEFI settings to enable booting from USB.
Step 9: Press any key when you see Press any key to boot from DVD… message on the screen.
Step 10: When you see Install Windows screen, select language, keyboard and then click Next button to continue.
Step 11: In the next screen, click Install Windows button.
Step 12: When you’re asked to enter the product key, enter valid 25-character product key and then click Next button to proceed to next step.
Step 13: On Where do you want to install Windows screen, select your Windows 8.1 drive or any other drive on which you want to install Windows, format the drive (if required), and finally, click Install button to begin installing Windows.
Step 14: From here onwards, you can follow on-screen instructions to sign in with Microsoft account and complete the installation.
Refer to full article from Microsoft about Update to Windows 8.1 from Windows 8