Window -> Package Manager
com.unity.entities
com.unity.rendering.hybrid
com.unity.platforms
com.unity.physics
Add package from git URL ...
enter package com.unity.entities
, com.unity.rendering.hybrid
and com.unity.platforms
Edit -> Projects Settings -> activate "Enter Play Mode Options
"
https://docs.unity3d.com/Packages/com.unity.entities@0.51/changelog/CHANGELOG.html
https://docs.unity3d.com/Packages/com.unity.rendering.hybrid@0.51/changelog/CHANGELOG.html
https://docs.unity3d.com/Packages/com.unity.platforms@1.0/changelog/CHANGELOG.html
https://docs.unity3d.com/Packages/com.unity.entities@0.51/manual/index.html