[Windows Central Question]

phone update is not working in mobile data it simply asks for wifi

[Harrie-S]

If what you want to download is more than 100 mb you need a wifi connection. You can not use cellular data.

[Athull]

You do need Wifi for all OS updates which are over 20mb in size. Same with apps over 100mb in size.
If wifi isn't available I believe u cld flash the latest software from nokia software recovery tool but that will wipe all the data in ur phone.
Alternatively u cld take it to a nokia care and they will do it for u but once again u will lose all data in ur phone.
Also if u can buy a software like connectify in PC which sets up a virtiual hotspot but for that money u cld just as well get a proper wifi router :)