How to update the existing records using Re-import (OOTB Import)Wizard in MSCRM 2011:
If you export a set of Accounts for example and select the option to
make available for update, save the file and then open the xml document
using Excel, you'll see the format by expanding the first few hidden
columns. You can see that there are some
unique values assigned that trigger the Update feature during import.
Step1: From Advance find take the view where the primay key( Account -account name) and also the required updated column (FAx) should be there.
Step2: Export the view but select the below option (MAKE THIS DATA AVAILABLE FOR RE IMPORT BY INCLUDING REQUIRED COLUMN HEADINGS) :
STEP3: Open the exported file (open with Xml editor)
Step 4: Update the fax column data in the above open excel with the data you required and save . Step5: Import the same sheet from importing wizard like normal way.