mysql_upgrade: 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/mysql_upgrade - from package: mysql55w-server
/usr/bin/mysql_upgrade - from package: mysql-server

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

On Debian possible default full paths are:

/usr/bin/mysql_upgrade - from package: mariadb-server-10.0
/usr/bin/mysql_upgrade - from package: mysql-server-core-5.5

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

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