Earlier today, we reported that you can now install Fast ring builds from the Redstone Development Branch on the Nokia Lumia Icon. While it wasn't clear at the time if users would be able to, you can use the Slow ring as well.
Any ring that you decide to choose will start you off with build 10586.107. Once that is installed, the Slow ring will get you build 14295.1005.
In short, Icon users have the same options that an officially supported device has, which includes the Release Preview, Slow, and Fast rings. As always, it's worth noting that Windows 10 Mobile does not come bundled with device-specific firmware, so the only difference between an "official" upgrade and the Release Preview ring is that you'll get cumulative updates a bit earlier than everyone else.