You can use the "Assessment Workflow" feature to select a preset period of time after which the assessment is automatically deleted. You can choose between complete deletion of all data or deletion of personal data - the remaining data is then used as part of the Insights.
Alternatively, you can also delete the assessment manually. To do this, click on the three dots to the far right of the relevant assessment in the assessment overview. Then select the option "Insights Archive" or "Delete". A pop-up window will open in which you must confirm the deletion by entering the assessment name.
You can find more information on deleting assessments here.