The Icon for that button is a Symbolic Icon.
It is named preferences-desktop-apps-symbolic and in Zorin OS, this symlinks to view-app-grid-symbolic. It is located in /usr/share/icons/Zorin/scalable/
To change it, create a new directory in your home folder named .icons. Then you can elevate to Root:
sudo -i
Open your file manager:
nautilus
Then copy the Icon Directory in /usr/share/icons/ that you are using to ~/.icons
Right click the newly copied file and change the ownership and permissions to You and not to Root.
Close the elevated nautilus file manager.
Open nautilus again as Home User. Navigate to ~/.icons/Zorin(-icon-Theme)/scalable/ and rename preferences-desktop-apps-symbolic to preferences-desktop-apps-symbolic-backup
Now, place the icon you prefer to use in that directory and name it: preferences-desktop-apps-symbolic
Open your Zorin Appearance app and change your icon set to the one in your home directory. To make this easier, you can change the name of the directory; For example, from ZorinBlue-Dark to ZorinBlue-Dark-home.