I previously imported data from an excel sheet into CRM that had around a dozen columns in it. At that time some of those columns were empty. Now that excel sheet has been updat
One to do would be to run an advanced find with the new columns plus any columns that you would need to identify the rows. Next, export to Excel all of the rows from the page and check the box to make the rows importable (see attached screenshot).
Finally, put the data for the columns into the appropriate columns and then import the spreadsheet.
This is probably the best way to update existing records through the data import utility.