Public beta testers can download the new update by going to the Settings app on the Apple TV and navigating to the Software Updates section under "System." "Get Public Beta Updates" will need to be toggled on, and once it is, the Apple TV will download the beta software.
Those who want to beta test the software can sign up to do so on Apple's beta testing website.
tvOS updates (aside from major releases) have historically been minor in scale, focusing on under-the-hood bug fixes and improvements rather than major outward-facing changes.
Apple provides little to no information on what's included in tvOS beta updates, so we may not discover any major changes after installing the software. There were no significant updates found in yesterday's tvOS 13.3 beta.