I think you can update phone only on Wi-Fi but you can try one thing, I think this should work, go to data sense app, in limit option choose unlimited, then check for phone update.
There are in total three updates including wp8.1 update. Two updates are necessary in order to download wp8.1 update which you will get after registering from preview for developers app.
Update 1: Approx 10 MB
Update 2: Again 10 MB
WP 8.1 update: 646 MB ( I am sure about this figure as I get notification of it because I was having less space on my phone).
So, in total you need approx 670 MB.