You can and this link to the support page has a list of compatible video cards. Now it also depends on which power supply you have. There were two. The 360W and the 500W.

thank you jocean. i was confused because i read that in other xps uhd machines a graphics card couldnt be installed without other major changes to the processor.

also, what's the easiest way to figure out which power supply?

thanks

@logicalcubical  - Sure you don't have an add-in video card (AMD or NVidia)?

Go to the Support page and enter your Service Tag (don't post it here). Follow the link to the factory configuration and see if the PSU is listed there and/or if an add-in video card is listed.

There are only a few things you'd need to do if you install an add-in video card:

1. Disable Secure Boot in BIOS setup before installing the add-in card
   
   
2. Install the card
   
   
3. Connect first monitor directly to the card. A second monitor can be connected to that card, if it supports more than one monitor, or a second monitor can be connected to one of the onboard Intel UHD Graphics ports
   
   
4. Install the drivers for the new card

Dell has 21 day return policy.

Restocking Fees: Unless the product is defective or the return is a direct result of a Dell error, Dell may charge a restocking fee of 15% of the purchase price paid less shipping and handling, plus any applicable sales tax, which would be applied through a total reduced refund or credit.

https://www.dell.com/learn/us/en/uscorp1/terms-conditions/art-us-return-policy

• 21-Day Return Period for Certain Hardware and Software Products and Accessories: Unless you have a separate agreement with Dell, or except as provided below, all hardware, accessories, peripherals, parts, and media-based software (provided that the software is unopened and still in its sealed package or, if delivered electronically, software that you have not accepted by clicking "I agree to these Terms and Conditions" button), may be returned within 21 days from the date on the packing slip

so, why doesn't dell honor their return warranty policy?
how can i get in touch with them?
why did i waste my money on a piece of junk dell?

@logicalcubical  You can install a different video card, even with UHD graphics, as long as the add-in card physically fits inside the case, and the PSU is big enough to support the card's power requirements.

There's a list of Dell's OEM versions of video cards that have been tested with this PC model in the setup guide. You don't have to buy one from Dell, as long as the retail version of same card is same dimensions and will work with the PSU installed in the XPS 8940. So what's the issue here?

Here's Dell's US Return Policy. It's only valid for first 30 days from invoice date, not actual delivery date.

Should the UHD be disabled after the graphics card is installed? 

@ChasCos  - Not necessary to disable Intel UHD Graphics when an add-in video card is installed.

Great. Thanks Ron.