Extract your downloaded firmware to C:\Program Files\Nokia\Phoenix\Products\RM-346\ . If the folder doesn't exist, create it manually.
Some users report that antivirus software can block flashing tools; consider disabling it temporarily if the installation fails. nokia+e71+rm346+latest+flash+file+install
Flashing will erase all personal files, contacts, and messages. check the box.
The latest official firmware for the RM-346 variant is generally or v510.21.012 , depending on your region. nokia+e71+rm346+latest+flash+file+install
If Phoenix doesn't see your phone, try the "Dead Phone" trick: Disconnect the USB, hold the Power button for 8 seconds until it vibrates, then immediately reconnect the USB.
If your phone is "dead" (won't turn on), check the box.