Regarding which version the phone should be upgraded, you can distinguish it by the follow way.
Due Due to the distinction of the hardware platform of the phone, if the DSP parameter of the phone with "M", then
UC902/UC903/UC912(G)(E) need to upgrade the fw900M.rom
UC924/UC926/ UC923/UC924(E)/UC926(E) nned need to upgrade the fw920M.rom 
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If the DSP parameter of the phone without "M", then
UC902 upgrade the fw902.rom, UC903 upgrade the fw903.rom, UC912 upgrade UC912(G) upgrade the fw912.rom
UC923 upgrade the fw923.rom, UC924 upgrade UC924 upgrade the fw924.rom and so on. 
, UC926 upgrade the fw926.rom.
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If the DSP parameter of the phone with "D", then
UC926U/UC926S/UC924U/UC924W/UC923U/UC921U need to upgrade the fw920U.rom
For the new models
UC501 upgrade the fw500M.rom, UC503(G) upgrade the fw500M.rom, UC505 upgrade the fw520M.rom, UC507 upgrade the fw520M.rom.
UC902S upgrade the fw910M.rom, UC921(G) upgrade the fw910M.rom
If you still can't distinguish, you can choose the corresponding file upgrade according to the interface prompt of the phone upgrade.
