I have selected the option to return to the Classic UI for document libraries via the option that was available when viewing a document library. Now I want to switch back to the New Experience UI, how do I do that?
I also wondered further about the session cookie making the classic user experience and it was cookie named "splnu" with value 0. By removing it and pressing refresh button it switched back to setting by admin that was the new user experience.
For convenience it is good to use a kind of cookie manager extension in your browser - I used the cookie inspector for the google chrome (and available as tab) in the chrome developer tools and it was really a piece of cake :)
- In your document library, choose Settings icon and then Library settings.
- Click Advanced settings and then scroll down to List experience.
- Choose New experience.
For more details check Switch the default experience for lists or document libraries from new or classic
I found the answer to my question here http://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_manage/how-to-switch-between-classic-and-new-library/b25035ec-21d0-4740-b7c7-1cf9ba3311bb.
The gist of this answer is that you must clear the browser cookie that indicates to use the new experience UI. The article states to clear all cookies, but I found I only needed to clear the cookies related to my SharePoint site. I was not able to figure out which cookie specifically needs to be cleared.