Importing Contacts/Accounts

Importing Contacts/Accounts

we have implemented Zoho in our company for B2B operations.  Our sales force have provided me with relevant customer lists that I have imported into the database.  We have a number of user defined fields that need filling in as we get the information (i.e perception to our product or test volumes)

I would like to have the facility where you can mass update those relevant fields based on information in a spreadsheet.  Using the import Contacts function does not seem to work because it just creates duplicate records, when what I want is do a mass update of 1 or 2 field in that record

Thanks for your help