If installing from off the asset store, such as git, itch.io or the carter games website. You’ll simply get a .unitypackage file. To import the .unitypackage into your project just double click it with your project open or use the import custom package option in Unity under
Right click in Project Tab → Import Package → Custom Package
Extension packages can be installed by simply double clicking the package file for the relevant extensions.
🔺 Updating to a newer version
If you are updating the asset from an older version it is best to delete the asset folder & do a clean install for the least amount of friction with the new version. If the update is just a patch you should be good to just import on top of the old files, but consider a clean install if the asset doesn’t work as intended or throws an unknown error.