Enrolling as an Apple developer allows users to create apps and costs $99 a year, users registered also get access to iOS beta’s, if you would like to enroll follow the guide below, if you would like a cheaper option click here.
Step 1: Before you can go ahead and download iOS 10 beta, you need to first enroll yourself into Apple Developer Program at https://developer.apple.com/programs/.
Step 2: Login to Account section of Apple Developer Program using your Apple ID: https://developer.apple.com/account/.
Step 3: Now you will need to register your UDID with Apple for the iOS device you want to install the beta on.
Step 4: From the main Account section of the page, Click on Certificates, Identifiers & Profiles.
Step 5: Once the UDID has been added, head to the Resources> iOS> Download section. You can find this page directly at https://developer.apple.com/ios/download/