update_firmware: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/update_firmware - from package: firmware-tools
/usr/sbin/update_firmware - from package: firmware-tools

You may install the required package with command yum install {package}

Error in other languages:
update_firmware: bevel nie gevind nie
update_firmware: comandă negăsită
update_firmware: comando não encontrado
update_firmware: commande introuvable
update_firmware: command not found
update_firmware: käsku ei ole
update_firmware: không tìm thấy lệnh
update_firmware: komanda nerasta
update_firmware: Kommando nicht gefunden.
update_firmware: kommandot finns inte
update_firmware: komut yok
update_firmware: nie znaleziono polecenia
update_firmware: níor aimsíodh an t-ordú
update_firmware: no se encontró la orden
update_firmware: no s'ha trobat l'ordre
update_firmware: opdracht niet gevonden
update_firmware: parancs nem található
update_firmware: perintah tidak ditemukan
update_firmware: príkaz nenájdený
update_firmware: příkaz nenalezen
update_firmware: команда не найдена
update_firmware: командата не е открита
update_firmware: 命令找不到
update_firmware: コマンドが見つかりません