update: 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/lib/mailman/bin/update - from package: mailman

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

On Debian possible default full paths are:

/usr/lib/mailman/bin/update - from package: mailman
/usr/lib/wims/bin/update - from package: wims

You may install the required package with command apt-get install {package}

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