Simple way to upgrade nokia c3 :
1. Nokia software updater.
If used Nokia software updater your phone can not flash phone in dead mode,must normal mode also use sim card,but not need firmware, this software worked with internet service.
2.Flashing use Phoenix nokia.
If used phoenix,must have firmware for nokia c3,better latest version,can download here,your must flash in dead mode.
I feel this post helpfull for someone wanna upgrade your nokia for all tipe BB5,for DCT4 just some model will support like Nokia C1-01.