ICNS is the macOS icon container - what sits inside an .app bundle. Since Lion it stores its images as ordinary PNGs at a handful of sizes, which is why reading one means picking the biggest member and writing one means rendering several.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop an ICNS file above and choose afterwards.