Go to Setup > App Settings App Designer
If you have upgraded to Premium App Designer, you will have access to change the Icons for each menu item. If you are not sure if you have Premium App Designer, please contact our Customer Success Team who will be able to assist you with your enquiry.
To change the Icon that appears next to a menu item, click on the Icon you would like to change, then select the Icon you would like displayed.
When you are happy with your changes, click on Save Changes at the bottom of the screen.