The best method for updating records with a Special Flag is to use the Import Center.

Before you start, open the CSV that stores the record data.  Add a new column to this CSV with the top cell labeled "Special Flag".  This column should be empty of any information but the title.

1.  Open the Import Center (Control > Import Center)
2.  Click on New Import
3.  Select your Category as Constituents Profile
4.  Select your Template as Individual
5.  Click Next
6.  For step one you are selecting the information that is used to match to a contact record.  In the absence of a Supporter ID, select Email
7.  Set the mapping from your CSV columns to the fields that should be updated.  First Name > First Name, etc

- Be sure to map your Special Flag column to the Special Flag (special_flag) field

8. Click Next
9.  On the next page you can set the default value for a field you've chosen to map.  Write in the value of the Special Flag you would like to use in the Special Field line's text box.  This flag should be unique.
10.  Click Next
11.  Be sure to set your duplicate handling to "Update existing Sphere records"
- Note: Be sure that the data you are importing is appropriate and can overwrite the data on your existing records.  If in doubt, set the selection to "Do not update exisitng records and log an error".  Check over the failed records after importing to compare data.

12. Click Next
13.  Give your import a name and click Finish

Once the import completes all records that were created or updated will have the Special Flag applied.