It changes according to the icon theme you are using, the file “start-here-symbolic” more specifically. To change it just place the icon file you wanna use (.svg or .png) to overwrite the existing one on the icon theme you are currently using.
Better to rename the old one first so you don’t lose it.
Check these links for more details:
http://antechdesigns.esy.es/change-start-button-in-brisk-menu/