1) Open the individual record and update it.
2) Use bulk edit and update multiple records in one go.
3) Export data from CRM, make changes offline and then re-import the data.
While following the Re-Import option we rarely get the following the error
The data has invalid column headings, so you will not be able to re-import it.
The issue actually has to do with duplicate display names. Since the export writes the display name as the column name, you cannot have duplicates because CRM doesn’t know how to match up the columns again when it re-imports the data.
In the scenario below, there is an issue with a duplicate status fields; the out-of-box status and a custom status field with the same name.
Good news is CRM 2015 validates the columns before exporting and gives a proper message now.
Hope this helps.