Your phone will wait a period of time for Wi-Fi. If Wi-Fi is not available, then it will prompt you and ask if you wish to use your cellular data. At this point, it will warn you exactly the size of the download when it gives you the option. You can either accept the download over cellular data or wait until you reach Wi-Fi. It's up to you.
I have done this on several upgrades of Windows 10 Mobile preview. It prompted me again last night, but I decided to defer for Wi-Fi at this time.
I can't see why couldn't you update throught your celluar data. Internet is internet. Altough it will take alot of your bandwith and may create aditional chargers. It is all up to you.
The biggest joke is it stopped my update updater on mobile data saying i am roaming. Of course I am outside but it should have given me an option. After all its my money. By the way, i don't pay anything extra while i roam. MS seems to assume that?
Yes you can update on cellular data. And obviously you need good connection speed for that. I did it today on cellular data. Once it said you are raoming, although i was not. Then i put my sim in other mobile and then again put it into mine. This time it downloaded easily.
The biggest joke is it stopped my update updater on mobile data saying i am roaming. Of course I am outside but it should have given me an option. After all its my money. By the way, i don't pay anything extra while i roam. MS seems to assume that?