Please! I Know what Windows Insider is and what slow and fast ring is, but I didn't know these options of "release preview" or "production" options exist. Also IDK how to be part of Window Inider, what do I need and how?
Alright allow me to help you out.
1). Navigate to the Windows Store and search for "Windows Insider."
2). Windows Insider will have a purple background, with a white phone and key. Download it.
3). You'll want to get on a PC and go to
https://insider.windows.com/ to register as a Windows Insider. There's no cost of any kind, it's just like setting up a Microsoft Account. In fact, you'll be using your Microsoft Account. Just click the big purple "Get Started" button to, ah, get started.
4). Once you've done that, go back to Windows Insider on your phone and make sure you're signed in with the Microsoft Account you used to become an Insider. If you're not than tap "sign in with a different account."
5). Now you'll need to hit "Get preview builds" near the center of the screen. This will bring up a "enroll" page with the list of rings. You should see "Insider Slow", "Insider Fast", "Insider Release Preview", and possibly "Production" (I doubt you'll see this one as you're still on Windows Phone 8.1 so don't worry about it if it's not there). Tap "Insider Release Preview" to highlight it than tap the arrow at the bottom of the screen.
6). This will bring up a "confirm" screen warning you that you might be installing dangerous and experimental software. As you've chosen Release Preview this really doesn't apply to you. Tap "accept" and move on. Your phone should now say "Configuring device..." and reboot.
7). Once your device comes back on navigate to Settings>Update and check for updates. You'll probably install a configuration update first. Just keep updating, checking for updates, repeat. You'll likely have three updates. Configuration, initial Windows 10 Mobile update, than another update to get you to the latest version. I suggest not touching anything until you've gotten to the latest Windows 10 Mobile update as early updates are more buggy.
8). Make sure all your apps are updated by going to the Windows Store before you do anything. Many of the apps will likely look very rough before they've been updated. They might not even work properly.
10). Set up your phone and enjoy Windows 10 Mobile.
If you need any more information let me know. Remember that your experience may differ. You may not have more than one update, and immediately jump to the latest Windows 10 Mobile update (.218) and your apps may be a more recent version. I'm not certain on that, as updates have been released since the last time I updated a phone. If you want to check your phone's software version go to Settings>System>About and make sure you're on the latest version (again .218).
Another thing: I personally suggest you factory reset your phone after you've finished updating to Windows 10 Mobile as this will likely provide a smoother experience and less growing pains but it's completely up to you and your experience should be just fine without a factory reset.