To delete a workout:
Select the workout.
Press the Delete button.
Confirm.
The workout is removed from your plan immediately, and also deleted from Intervals.icu (and therefore from your devices).
Can I undo a delete?
No — deleting a workout cannot be undone, so the confirmation is worth reading. If you deleted something you needed, recreate it: ask LeCoach in chat ('recreate the threshold workout I just deleted for Thursday') — it knows your plan and builds an equivalent workout in seconds.
Deleting many workouts
To clear a whole stretch, ask LeCoach in chat ('remove all workouts next week') — you'll get a preview before it's applied. To remove the entire plan, see What happens if I delete my training plan?