Apple's Shortcuts app lets you make and use custom icons for any app on your iPhone or iPad, but it won't actually replace any icons. If your goal is to change the official icon for the app itself without any redirects, it's possible, but only for some third-party apps found in the App Store. While the Shortcuts option works well and gives you complete control over an icon's design, the custom icons you make are just shortcuts to the real apps you've (hopefully) hidden from your Home Screen. That clutters your list of shortcuts and gives you duplicate entries for the app in your App Library... more
){kind=link}