Importing the package
To get the asset you’ll have to press the Add to My Assets button. Doing so will add the asset your account so you can access it in which ever version of Unity you want that the asset is supported in.
From the asset store you can then press the Open in Unity button. Doing so will open Unity on your system with the package to use.
When in Unity you can import the asset via the package manager, found under Window → Package Manager. The window will have to option to filter to the packages, from here you can select My Assets. You may need to login to your UnityID to see your assets. Once the packages appear, the Leaderboard Manager will appear on the list. Here you can download the latest version of the asset and then import it via the button in the bottom right of the Package Manager panel.
Once you press the import button, a package window may appear with information about what the package contains and options for you to choose what to import. You should import the full package and then remove and demo elements if you don’t want them. You can see an example of the package import window to the left. When ready, just press import and the package will import into project, the editor will reload and you will be good to go.