Link rot; tutorial is now here: https://gripfastistech.com/index.php/blog/66-how-to-change-the-brisk-menu-icon-in-ubuntu-mate-18-04.html
TLDR; change the /usr/share/icons/Adwaita/places/start-here-symbolic.png that most closely matches your panel size, e.g. 4848, 64x64, etc.