Follow the below steps to install Public Beta version of iOS 14 and iPadOS 14:
1: On your iPhone open Safari and visit the Beta Program page of Apple
2: Click on the Sign up option and sign in with your Apple ID and accept the terms and conditions of beta programs when prompted
3: You need to make sure, you have backup of all the data and then click on Download Profile and follow through the process as prompted.
4: Restart the device if required
5: Now Go to Settings > General > Software update and download the beta profile