First of all, copy all PNG files on the downloaded folder to a desirable path.
Open Cisco Packet Tracer on your computer or any of your projects.
If you don’t have a project yet, just create a new project and make your desirable design topology.
I already have a project and I am going to change the Server Icon, you may understand after seeing the difference.
Double click your chosen device.
Bellow the device you may see a button on the left saying “Customize Icon in Physical View” and on the right saying “Customize Icon in Logical View” click on the right button (Logical View) and browse to the saved folder.
There are more than 290 PNG files on that folder, choose the desirable icon to change it.
You may want to use search to save your time.
Just type Server on the search bar to change the Server Icon.
Don’t forget to save the project so you don’t lose the icons.