Windows 8.1 Start Button Changer is a freeware from door2windows which lets you easily change the start button in Windows 8.1.
How to use:
Download and run the app.
Click the Change button.
Browse and select the image which you want to display as the new start button. The app will automatically resize the image for you so it is not necessary for the image to be of specific dimensions.
Note that a start orb bitmap made for Windows 7 won't work here. Windows 7 start orb bitmaps contain images for the default, hover and pressed state in one image but in Windows 8.1, the default and hover/pressed images are stored separately.
If you want to convert a Windows 7 start orb for use in Windows 8.1, use: Windows 7 To Windows 8.1 Start Button Converter
Click Open. Windows Explorer will restart automatically and the new start button will be displayed.
To revert to the original start button, click the Restore button.
Right-clicking the Change button will provide you the options to change only the default image or only the mask image (hover and pressed).
Credit goes to Angus Johnson for the resource updater.