How to delete rows from relational database
One simple way to do this is to extract the rows from those tables into temporary tables then truncate the source tables.

Example:

-- Load the top 500 UUT_RESULT records into a temp table
SELECT TOP (500) *
INTO #UUT_RESULT_TEMP
FROM UUT_RESULT
ORDER BY ID

-- Load the related STEP_RESULT records into a separate temp table
SELECT *
INTO #STEP_RESULT_TEMP
FROM STEP_RESULT
WHERE UUT_RESULT IN (SELECT TOP (500) FROM UUT_RESULT ORDER BY ID)

-- Remove the record
TRUNCATE TABLE STEP_RESULT
TRUNCATE TABLE UUT_RESULT

-- Load the records back in
INSERT INTO UUT_RESULT
SELECT * FROM #UUT_RESULT_TEMP

INSERT INTO STEP_RESULT
SELECT * FROM #STEP_RESULT_TEMP


Using TRUNCATE will help speed things up as this is a minimally logged operation.