melanie
commented
3 years ago If you want to update the images yourself, it's pretty easy!
From any scene, click this tiny little arrow on the edge of the UICanvas prefab in the scene, which takes you to the prefab editor:
In that view, scroll down the hierarchy until you find MainMenu:
It's disabled by default - so look over in the Inspector panel, and check the topmost checkbox:
Then it's be visible, and you can navigate down through each button layer to find the button's image, and swap it out with the new art.
MazurkaLFG
slight variant 
The Main Menu has two icons from a ui kit I purchased, and one I made (gee, can you guess which one?)
We should probably do our own icons for all of these (including a unique one for Quit, and probably one for Settings, too).
They don't have to look like objects, like the current ones do - they could be one- or two-color icons, or whatever. The ones in there now are around 16x16.