How to change app icons on a Mac

macOS lets you replace almost any app's icon with a picture you like. It takes about a minute per app by hand. Here's the manual way, the catches nobody mentions, and the fast way.

The manual way: Finder's Get Info

Get an icon. Use a square PNG, ideally 1024×1024 with a transparent background, so it looks right next to other Dock icons. Every icon in the Iconic catalog is already in that format.
Open Get Info. In Finder, go to Applications, click the app once and press ⌘I.
Drop the icon on. Drag the PNG onto the small icon at the top left of the Info window. Alternatively, open the PNG in Preview, press ⌘A then ⌘C, click the small icon in Get Info and press ⌘V. macOS may ask for your password.
Refresh the Dock. Quit and reopen the app. If the Dock still shows the old icon, open Terminal and run killall Dock.

To go back to the original, open Get Info again, click the small icon and press Delete.

Why it sometimes doesn't work

Apple's own apps. Safari, Mail, Messages, Music, Finder and the rest live on the sealed, read-only system volume, so Get Info can't touch them. Iconic works around this by building a tiny wrapper app that points at the real one and carries your icon, then swaps it into your Dock.
Apps that paint their own Dock icon. Arc, Warp and a few others draw their Dock tile themselves and reset it every launch. Use the icon picker in their own settings (Arc → Settings → Icon, Warp → Settings → Appearance).
App updates. An update replaces the app bundle, and your custom icon goes with it. You have to reapply it after every update, or let something do it for you.
macOS Tahoe's gray squircle. macOS 26 forces every icon into a rounded square and shrinks the rest onto a gray tile. See how to fix squircle icons on Tahoe.