It happens. You install the latest and greatest, but it doesn't work, or you hate it, or something that you just really need isn't available. So how do you go back? Well, it isn't pretty, but it can be done.
First, you'll need to make sure you have a few pre-requisites satisfied. You'll need a Windows 7 or newer computer, with at least 4GB of storage space, and a USB cable to connect your phone to the computer. You'll also need a Lumia phone, this recovery tool was developed by Nokia, and won't work on other brands (but you can't install WP 10 on other brands yet anyway).
Then, you'll need to be aware of the caveats. Using the Recovery Tool is going to wipe your phone, and take a while:
Important! Using the Software Recovery Tool will erase all content stored on your phone, including apps and games (along with app data and game progress), text messages, call history, music, photos and more. If possible, you should back up your phone first (Settings > backup). You will be able to restore a backup once the recovery is complete.
Note: The recovery process may take some time depending on your internet connection speed, and you cannot use your phone during the installation. The latest approved version of the software in your phone and model will be installed.
OK, so you know the risks, and you're ready to recover your Windows 8.x or 10.x phone (if you're still in the dark ages running Windows Phone 7 on a Lumia device and need to recover your phone, there is a tool for that, too). The first thing you'll need to do, on that computer you have ready, is to download and install the Windows Phone Recovery Tool. Next, follow the instructions, and be prepared to wait:
Download and install Windows Phone Recovery Tool to your computer. If possible, make sure that your phone's battery is fully charged.
When the installation has finished, start the application and the main screen will appear:
If your phone is not automatically detected, disconnect all phones from your computer and press My phone was not detected at the bottom of the main screen.
If prompted, select the phone manufacturer.
Connect your phone to your computer with a compatible USB cable.
Wait for your computer to detect the connected phone and follow the screen instructions. If the phone is not connected within one minute you can try to restart your phone: Keep USB connected, press and hold the Power and Volume down keys simultaneously. Release the pressed keys when phone vibrates.
Press Install software to start installing the software to your phone.
After that, you may have to re-install updates, including to the most recent "GDR" available for your phone, and your apps, but your phone will be back to a state from before you started messing with it. Good luck!