According to Apple's developer website, the second AirPods Pro firmware update adds support for Conversation Boost, a feature that was missing from the first beta.
Conversation Boost is an iOS 15 feature that uses the beamforming microphones in the AirPods Pro to boost the volume level of conversations, which is ideal for those with mild to moderate hearing challenges.
The initial AirPods Pro firmware beta added support for FaceTime Spatial Audio and Ambient Noise Reduction, two other AirPods Pro features that are coming in the iOS 15 update.
Only registered developers are able to install the AirPods Pro firmware, and the installation process is rather complicated and requires Xcode. We have a walkthrough on installing the profile for the beta, but as with all over-the-air firmware updates, it takes some time for new software to be seeded to users.