Apple just released a public beta for iOS 8.3 and anyone can sign up to check it out. Here's how to do it.
If you're already a part of Apple's Beta Software Program from the Yosemite pre-release, you're good to go. Otherwise, you can sign up over on the Apple Seed site.
Once you sign up, head to the Apple Seed site from your iOS device, log in, and enrol in the iOS public beta program. Once you do you'll be instructed to download a certificate. Once downloaded, the iOS 8.3 update will show up in Software Update.
It seems like the beta is still rolling out, so if you don't see it right away, check back later today. There aren't a lot of major improvements in iOS 8.3, but you should get support for wireless CarPlay, an upgraded Emoji keyboard, and a few other tweaks.