Many scripts were renamed or removed and Unity Packages do not handle these changes very well. After version 1.0 has been removed version 1.1 can be imported.
@liquidsht By removing version 1.0.X we mean removing the folder containing the package within your Project.
So remove the "Assets/Opsive" folder or if you have other Opsive Assets in addition to UIS remove the "Assets/Opsive/UltimateInventorySystem" folder.
Make sure to make a backup of your project before updating and do not Save or close Unity between the moment you remove the folder and import the new package
There are a lot of changes since between v1.0.X and the latest version so make sure to watch the new video tutorials and read the docuementation