Before you submit your videos, you can watch them back and check if they're the right ones. If you realise it's the wrong video at that point, hover over the video and click on the '...' icon. Then select 'Delete'.
If you realise it's the wrong video after you've hit 'Submit my videos', you need to message us through the chat bubble on the bottom right-hand corner of the screen. Someone at Vidsy will help you out from there.