Why won't my MacBook Pro let me change app icons?

So if you look at the address bar you will see that I am running a old macOS, and most of the app icons are out of date. I want to replace them, I have all the images of the updated icons. I know how to, but the OS won’t let me, is there an alternative way then pasting the icons on top of each other? Because that doesn’t work.

Thanks in advance!

[Re-Titled by Moderator]