There are 4 open connections to the database. This may cause the upgrade to fail and leave the database in an
inconsistent state. We strongly recommend you to close all open connections before proceeding with the upgrade.
I am using windows server 2012 r2
How do I detect/close these open connections?
will show you the current connections. You can use kill 57 etc. where 57 is an example SPID that you want to disconnect. However, they may reconnect immediately if they are connecting through an app. Also, the impact of forcing connections to close on those users should be evaluated.
Ideally, if your users are connecting through an application, for example a web application, shutting down the IIS would be a good option.