Click on the to-do you’d like to remove from your checklist. The detail view will open for that to-do. At the bottom of the detail view, click on “Remove item”. The to-do will no longer be in your list.
If you want to bring that to-do back into your checklist, you can find it in the “Add items” tab. Simply click on the to-do and click “Add to my checklist”. The item will be added back to your checklist in timeline order, according to its due date.