Go by the values in the clustered index.  For example, if the table is clustered on a datetime column, use that as the value for the UPDATE.

Here's a shell script that lets you specify a starting and max value, batch size to update and total number of rows to update.


SET ANSI_NULLS ON;
SET ANSI_PADDING ON;
SET ANSI_WARNINGS ON;
SET ARITHABORT ON;
SET CONCAT_NULL_YIELDS_NULL ON;
SET NOCOUNT OFF;
SET NUMERIC_ROUNDABORT OFF;
SET QUOTED_IDENTIFIER ON;
SET XACT_ABORT ON;

DECLARE @batch_size_of_update int;
DECLARE @datetime datetime;
DECLARE @max_datetime datetime;
DECLARE @min_datetime datetime;
DECLARE @rows_updated int;
DECLARE @total_rows_updated int;
DECLARE @total_rows_to_update int;

SET @batch_size_of_update = 20000;
SET @total_rows_to_update = 200000;
SET @min_datetime = '19000101';
SET @max_datetime = '20171231';
SET @rows_updated = 0;
SET @total_rows_updated = 0;

PRINT 'Start time = ' + CONVERT(varchar(30), GETDATE(), 0) + '.';

WHILE 1 = 1
BEGIN
  SELECT TOP (@batch_size_of_update) @datetime = datetime
  FROM dbo.table_name
  WHERE datetime >= @min_datetime AND
   datetime <= @max_datetime
  ORDER BY datetime;
  UPDATE TOP (@batch_size_of_update) tn
  SET whatever = 'whatever'
  FROM dbo.table_name tn
  WHERE datetime >= @min_datetime AND
   datetime <= @datetime;
  SET @rows_updated = @@ROWCOUNT;
  IF @rows_updated = 0
   BREAK;
  SET @total_rows_updated = @total_rows_updated + @rows_updated;
  PRINT 'Total Rows updated = ' + CAST(@total_rows_updated AS varchar(10)) + '.';
  PRINT 'Last datetime updated = ' + CONVERT(varchar(30), @datetime, 120) + '.'
  IF @total_rows_updated >= @total_rows_to_update
   BREAK;
  SET @min_datetime = @datetime;
  IF @total_rows_updated % 200000 = 0
  BEGIN
   CHECKPOINT;
  END; --IF
  IF @batch_size_of_update >= 10000
  BEGIN
   IF @batch_size_of_update <= 25000
    WAITFOR DELAY '00:00:00.125';
   ELSE
   IF @batch_size_of_update <= 50000
    WAITFOR DELAY '00:00:00.250';
   ELSE
    WAITFOR DELAY '00:00:00.500';
  END /*IF*/
END --WHILE

CHECKPOINT;

PRINT '';
PRINT 'End time = ' + CONVERT(varchar(30), GETDATE(), 0) + '.';