apt update

manpage

執行

$ man apt

可以找到一段說明

update (apt-get(8))
    update is used to download package information from all configured sources. Other commands operate on this
    data to e.g. perform package upgrades or search in and display details about all packages available for
    installation.

help

執行

$ apt -h

可以找到一段說明

update - update list of available packages

指令範例

執行

$ sudo apt update

然後執行，觀看有哪些套件需要更新

$ apt list --upgradable

再執行下面指令，更新套件

$ sudo apt upgrade

或是執行

$ sudo apt full-upgrade