RowNum For T-SQL (Not TOP #)
Have a look at MS Docs about it:

Limitations and Restrictions
When TOP is used with INSERT, UPDATE, MERGE, or DELETE, the referenced rows are not arranged in any order and the ORDER BY clause can not be directly specified in these statements. If you need to use TOP to insert, delete, or modify rows in a meaningful chronological order, you must use TOP together with an ORDER BY clause that is specified in a subselect statement. See the Examples section that follows in this topic.

UPDATE TOP(500) instances
SET thing = 'new thing'
WHERE Something = 'SomethingElse';

Or
UPDATE instances
SET thing = 'new thing'
FROM (SELECT TOP (500) Id
      FROM instances
      ORDER BY Something) AS t1
WHERE instances.Id = t1.Id;

Or you can update the derived table directly without a join:
UPDATE t1
SET thing = 'new thing'
FROM (SELECT TOP (500) *
      FROM instances
      ORDER BY Something) AS t1;

An example of this approach can be found at dbfiddle.