The peppy new voice you're referring to is really just the existing voice that has been improved via the use of deep learning so that it has a smoother more natural sound than previously.
This is not something you have to select. Apple started introducing these changes as part of iOS upgrades, starting with iOS 10 and continuing with iOS 11. These changes have also been deployed to macOS and in fact with multiple languages (not just English!)
If you're interested, you can refer to Siri On-Device Deep Learning-Guided Unit Selection Text-to-Speech System. This is, however, a rather technical article.
In summary, there is nothing for you to do to get the benefits of this peppy new voice.
Just to clarify, there is no new voice option to get the benefit of Siri's improvements. That is, it's not a new voice, it just that Siri's voices have been (and continue to be) improved so that they have a smoother more natural sound.
To quote directly from Apple:
For iOS 11, we chose a new female voice talent with the goal of improving the naturalness, personality, and expressivity of Siri’s voice. We evaluated hundreds of candidates before choosing the best one. Then, we recorded over 20 hours of speech and built a new TTS voice using the new deep learning based TTS technology. As a result, the new US English Siri voice sounds better than ever.
Source: Deep Learning for Siri’s Voice: On-device Deep Mixture Density Networks for Hybrid Unit Selection Synthesis
For ease of reference, below are some audible examples you can listen to to compare how Siri's female English voice has changed over time. In each example you will hear the phrase spoken three times by Siri to compare how she sounds from iOS 9 to iOS 10 to iOS 11.
“Okay, I set up your meeting with David tomorrow. Shall I schedule it?"
“Always close the barn door tight.”