If the above doesn't work, then have you tried fully deleting the NOW App off your Samsung Smart TV, then download a fresh NOW App, then unplug the mains power to the television for 2 minutes, then switch on the television and sign back into the My Account section of the App.
I don't own a Samsung Smart TV, but i had to do something similar this week on the NOW App on my Fire Stick, plus i also removed the cache in my Fire Stick NOW App together with the above to get rid of the update message from my screen and for the NOW App to work again.
I have the same problem on my Samsung LS32AM700
Now TV requesting firmware update.
I downloaded latest update onto usb and updated tv.
I've also reinstalled NovTV app (can't delete because it's a basic app that came supplied)
Switched tv off for 2 minutes
Still get request for firmware update!
Somebody else also reported the same problem on NOW Twitter yesterday.
I don't think NOW Twitter resolved it from the tweets that i read between the two parties.
If nobody else on the forum knows how to resolve it, then maybe pop over to NOW live chat for further assistance & help.
Out of interest what is the model number of your Samsung Smart TV ?
You could try a full factory reset, but this would be a pain in the backside and i don't know if it would help anyway with the problem of the NOW App.
I have the same problem with a new Samsung tv, NOW app was working for a good few weeks then stopped, went through NOW tv chat and Samsung. Told had to down load a file to my old now stick to a pc then upload this I think to the tv, just didn’t make sense. So now can’t use now tv I am paying for so will cancel membership. Quite angry about!