@Mihir : According to this website of Microsoft for Windows 10 mobile , Due to large size of updates, they recommend to download through WiFi. However I cannot tell the exact size of Windows 10 on mobile,
I would recommend not to upgrade. There is still battery life issues, subpar camera quality compared to 8.1 and on top of that enjoy the message you charging your phone slowly error message despite using ms official charger.
If upgrade advisor doesn't update the work around is going from insider preview select third ring then opt out. You will then be on latest stable release along with the above issues I highlighted. That's why I went to back to 8.1 and said hello again to two days without charging my phone
Thank you for the comment on the diminished camera quality. I probably would have flung the phone out the Window before realizing I could have reverted back to 8.
@cruelsister you will need the recovery tool if you had upgraded to win10.
Trust me camera was one issue but the battery was worse.
They have improvements under the hood but sadly I'm on the opinion for once that 8.1 is currently better than 10 for wp. And they did delay its release for a reason. Luckily so far auto updates on 8.1 will not force w10