If you are using Start 8 or Start 10 you can change it there,. Very easy. Also there are a lot of start buttons for download on WC.
If you want to make you own and change it in skinstudio. Open SKS. Open the skin you want in it. Go to Edit startmenu and taskbar. Click on Edit Horizontal taskbar. Go to the bottom and you will see Start button. Click on Button and add you new one. I would suggest you use the button size that is being used. If you click on Edit Image it will open that image in your graphic program.
The easiest way is to change it through Start 8 or 10. https://www.wincustomize.com/explore/start_buttons
