Apple releases what it calls Safari Technology Previews. This is available now for users of macOS Big Sur. You can install Safari 15.0 on your Mac and it will install as a second app, alongside the existing public version of Safari. You can download the technology preview from Apple's mini site.

You will have both Safari 14 and Safari 15 Technology Preview on your Mac. You can test out the new features as much as you want, and if it gives you issues, switch back to the public build and remove the Technical Preview without having any risk.

Otherwise, sit back and be ready for a new Safari experience and much more coming soon.