After installing the Windows 10 tech preview on your Windows Phone, you may want to go back to Windows Phone 8.1 - in this video I show you how if you have a Lumia Phone.
YOU WILL LOSE ALL PERSONAL DATA ON YOUR PHONE!!!
So, make a backup if there are things you want to keep.
Go to this page
Download the Windows Phone Software Recovery Tool (for Lumia Phones running Windows 8 or newer)
Read all disclaimers on the page!
Install the software, launch, connect your phone to your computer. The Recovery Tools should detect your phone model, select it.
In my case I already had the latest firmware/software, but that's fine, go ahead and re-install it so it sets your phone to use the LATEST APPROVED software version for your cellular carrier, in my case, Windows Phone 8.1
The process can take awhile, so relax, and let it run. After the software has been installed, you will need to setup your phone just like when it was new (assuming you don't have a backup you restore)
You can easily get back all your apps by going to STORE, click the ellipses (...), My Apps, press the Select button, and then place check marks on all the apps you want to install. Press download, let it run.
Be sure to read all disclaimers before running the Recovery Tool, and make sure you use the correct version for your phone.