If you need to delete an assessment collection, all you will need to do is go to DATA > select keystage and cohort > GRADES > Uploads > Assessments and click on the assessment collection you want to delete. As long as the collection doesn't contain any grade files, you should have the option to just click the red 'Delete Collection' button.
However, if you find that the 'Delete Collection' button is greyed out, it is usually due to the collection having contained Attitude to Learning judgements that were not deleted before the grades file was deleted. As the Attitude to Learning tabs can't be accessed when there are no grades uploaded into the collection, the collection contains data that you can't delete.
All you will need to do is upload a grade file into this assessment collection. Any grade file will do as it is just to allow you to access the AtL judgements. Once this file has been uploaded, you will be able to click on the AtL tab(s) and then click the red 'Delete Judgements' button.
You will need to do this for each Attitude to Learning category you have set up, then you can click on the 'Grades' tab and delete your grades file by clicking on the red X icon. Once the AtL judgements and grades files have all been deleted, you should then be able to click on the 'Delete Collection' button.
Thanks for reading!