Computer running Windows 8 and want Windows 8.1, can only be upgrade it through the Windows Store. And if you got multiple Windows 8 computers, then each computer will need to download the same upgrade individually, which results in waste of time and for some it might be a issue if they have a bandwidth cap from their ISP.
After looking up on the internet, I've found a nice workaround (credit to the person figured it out), and I'll share it with you here.
Before getting started, you will need a Windows 8 Product Key. Computers with Windows 8 preinstalled, the key is inside the bios. These key may or may not work, but you can still try it out.
To extract the key out, use a Key viewing utility, like KeyFinder or other similar programs.
1. Go to Microsoft's Windows 8 Upgrade site
http://windows.microsoft.com/en-us/windows-8/upgrade-product-key-only
2. Download and save both setup (Install Windows 8 and Install Windows 8.1) files to your computer.

3. Go to the location where the files are saved.
4. Run Windows8-Setup and let it load

5. Type in your Windows 8 Product Key

6. Setup will analyze the key and then begin downloading.

7. Once it starts to download, click on the X on the top right corner and cancel it (no need to finish downloading).

8. Run WindowsSetupBox

9. Windows 8.1 will automatically beginning downloading. This may take a while so be patient

10. After it's done downloading, it will provide you with 3 options

11. From here on, you can create a ISO media, a USB bootable media or run from your Desktop.
For ISO media, setup will ask for a location to save the image. After that, burn it to a blank DVD.
1. Select Install by creating media and click Next

2. Click on ISO file and click save

3. Save to a location and label the iso file

4.Setup will being creating a image

5. Image creation complete

6. Go the location where the ISO image is saved and right click to open up Window's image burning utility.

7. Click burn to start burning from ISO to a blank DVD media

For USB media, setup will ask for a USB flash drive of at least 3GB. And setup will automatically create the USB media for you.
1. Click on Install by media

2. Click USB flash drive, then click next

3. Insert a USB flash drive

4. Format USB flash drive

5. Creating Windows 8.1 on a USB flash drive

6. USB media complete

Note: Backup data first if there is any on the USB drive.
Product key is made up for this tutorial, it's not a real key.
The version being downloaded corresponds to the product key that was used
Windows 8 Professional key = Windows 8.1 Professional
Windows 8 Core key = Windows 8.1 Core
Setup also looks at the platform your system is currently running.
Current OS is 32bit = Setup will automatically download Windows 8.1 32bit.
Current OS is 64bit = Setup will automatically download Windows 8.1 64bit.