RowNum For T-SQL (Not TOP #) Trying to run an update, but I want to step through it as it's quite a bit of data and don't want to blast it all at once.

In oracle it's a bit easier to select the rows I want because you can include it in the WHERE clause, but T-SQL you include it in the select. I'm wanting to write it something along the lines of this:

Update Instances
set thing = 'new thing'
Where rownum&lt;500


Basically want to do 500 at a time, is this syntax possible in TSQL?
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.