Hi Malinda
If you are updating the custom user field using Upload Users then this will be the same process as what you are used to. The column in the csv file will be profile_field_xxxxx where xxxx is the unique shortname of the custom field.
If you are using Totara's HR Import then the column name needs to be customfield_xxxxx Also note If the custom field type is date format then the data in the field needs to match the format defined under Location > Location settings > CSV import date format.
The value in the rows below it need to match one of the menu values from the field.
Regards