Normally you would be looking in settings or in right mouse click and properties but they are just not present.

Then you might want to look at the dconf database. Here more technical info about dconf.
In order to open dconf in a graphical application we install dconf-editor (if not yet installed).

We found lots of interesting things to change but not the icon.

Then there is only one way to change it and that is in the icon theme you are using.

We need to change the following icon :

start-here-symbolic.svg in folder places

Do not forget to delete the icon-theme.cache.

Follow the tutorial and will become clear.