To use the beta versions, head to Pterodactyl, and under the startup tab change "latest" to the specific beta version. For example, "2.0.0-beta". You will need to reinstall your server by clicking on the "Reinstall Server" button under the Settings menu after you change the version for the new version to download.
You can automatically use the latest beta version by using the version "latest-beta". When no beta versions are available, this will install the latest stable version.
Your beta version may differ, please check which version you would like to use.