It's easy to update contact data in Impactive by simply re-importing contacts with their relevant data added or corrected.
Impactive uses phone number as the unique ID for contacts, so if you upload a contact with the same phone number as they already had within Impactive, a duplicate contact will not be created - the same contact will simply be updated with whatever new or corrected data is imported.
(Note that this is true for both contacts imported manually via CSV, or imported via integration with VAN/EveryAction, etc.)
To give a very simple example - let's say that I originally imported a contact John Smith with a simple CSV like this:
first_name | last_name | phone |
John | Smith | 1234567890 |
A few days later, I realize that John's name is actually spelled 'Jon', so I need to update the contact. Easy! I just need to re-import the CSV with the information corrected, using the same phone number for this contact - Impactive will match on that phone number as the contact's unique ID, and will thus update the data for that phone number accordingly:
first_name | last_name | phone |
Jon | Smith | 1234567890 |
After importing that second CSV - in the same way I normally import a CSV contact list, via Contacts > Lists > + Import List - Jon's contact record will be updated with the new spelling of his name.
Note that this method of updating contacts by re-importing them using the same phone number is also the way that an administrator can manually add tags and custom fields to contacts (without filling a report via the volunteer interface) - more on that here.