A few more steps...
Archive a backup
Please note that you are about to install beta software. Always back up your data and files before installing beta versions of iOS. We recommend that you do a backup using Finder. Select your device from Locations in the Finder sidebar. Create a backup then click Manage Backups to Archive your backup.
Note: If you have not upgraded to macOS Catalina or later, you can archive a backup using iTunes. We recommend that you do an iTunes backup and archive it before installing the iOS developer beta.
From your iOS device, go to BetaProfiles.com to download the configuration profile. Your iOS device will notify you when the profile has been downloaded. Open Settings, tap the profile and follow the onscreen instructions to allow installation.
Update your iOS device
Tap Settings > General > Software Update to install any available beta software.