You can change your mind and return it with in the 14 days post-delivery regardless. Unless faulty this is usually at your expense but their FAQ says:
What are the Effects of Cancellation – Refunds and Returns?
- Refunds will be processed as soon as possible and you will usually be refunded within 14 calendar days of receipt by Dell of your cancellation notification.
- Once you have notified Dell of your wish to exercise your right of cancellation, Dell will arrange for collection of the Product(s) or provide instructions and details for you to return the Product direct.
- You must return Product(s) in their original condition and within 14 calendar days of your cancellation notification, unless Dell provides a later collection date.
- Dell may charge you for any damage caused to the Product(s).
- Where Dell arranges collection, return shipping will be free, unless Dell notified you otherwise. However, Dell may withhold payment of your refund pending receipt of the returned Product (in its original condition).
So they might pay for it!
Their Premium Panel Guarantee covers 1 bright pixel. It does not cover dead pixels per se, however if it arrives with a cluster of dead pixels then I expect they will accept a fault, although of course you are still within your rights to return it anyway if within the 14 day period if they refuse a RMA request.