Install by downloading the whole repository and opening it as a Unity Project
To add it to an existing project.
Using the Package Manager: "Add package from git URL" first add UnityGLTF: https://github.com/KhronosGroup/UnityGLTF.git (unless your project already contains it) Then add this package: https://github.com/icosa-foundation/open-brush-unity-tools.git#upm