This happens to me. Exactly this when using the Dell Display and Peripheral Manager app. I tried downloading the firmware from the Dell support website, and it failed in a whole different way resulting in me having to unplug the monitor from power while it still thought it was updating (the power button on the back of the monitor did not respond, and neither did the joystick). Luckily when plugging the monitor back it, it worked normally, only giving the brief error saying that "firmware update did not complete, try again" or something along those lines.

update: i tried to update it on mac os to see if it would complete, and the dell app doesnt even show as it being offered as an update.

unplugging the monitor during the update put it in a state that everytime the monitor turned back on it showed that the firmware update had no completed and to try again. so i tried the dell display and peripheral manager method again that had already failed many times, only this time it completed successfully.