The following directions are for Reuters Datalink

Updating the Local Data Files follows a similar format as to creating the local data securities.

Securities in the US Database are set up by Symbol not Name

Open The Downloader
Go Tools - Update Symbol Database (Generally updated around the 7th of each month)
Once completed, Close The Downloader and Reopen
Go to File - New - Security
Browse to the first folder you wish to update
Click on lookup
Go to the Group & Type for the folder your updating 
(i.e.) Updating c:\MetaStock Data\Stocks US\A-D
You would select the Type Stocks - Common
You would select Group - All
For symbol you would type A,B,C,D
Click on Select All
Click on Ok
Click on Create
Browse to the Next folder
Click on Lookup
Repeat remaining steps until finished
On your last folder instead of clicking create, click on OK