How to: Select a Company
You may have to change the company that you are working with. The companies that you work with can be stored in the same database or in different databases.
To select another company in the same database
On the Application menu, choose Select Company.
In the Company field, select the company that you want to work with from the list.
You can also open the Select Company window by choosing the company name in the status bar at the bottom of the program window or by pressing Ctrl+O.
In later versions of the NAV Web Client you can change the company in the web application by selecting the user profile button, "My Settings". Note, you must sign out and back in after changing.
https://msdn.microsoft.com/en-us/library/hh179433(v=nav.90).aspx