You are not supposed to be able to refresh the access token without the user's intervention. The system was designed this way in order to protect the user's data from being accessed indefinitely.
Linkedin wants to ensure that you cannot access the user's data unless they are actively using your application. To me, if a user doesn't visit your application within 60 days, it means that they aren't using it, and you should not be able to access that data.
I hope this helps.