This article shows you how to change back and forth between Lightning and Classic views.
Changing to Classic View
To change to Classic View, click on the Profile Icon and select Switch to Salesforce Classic.
The Classic View provides exactly the same data as the Lightning View, the only difference is the user interface design.
Changing to Lightning View
To change to Lightning View, click on your name at the top of the interface to open the dropdown menu and select Switch to Lightning Experience.
The Lightning Experience is the new, responsive user interface for Salesforce.