The raspi-config tool helps you to configure your Raspberry Pi; several settings can be changed with this tool without having to know the correct commands to use. It is written as a bash script, run in a terminal window, and uses whiptail (whiptail is a “dialog” replacement using newt instead of ncurses, see “man whiptail”) to create the windows, menus and messages. Some changes require “administrator” permissions, so the tool must be run in a terminal with:sudo raspi-configThis can be run from the command line or from a terminal window if using the GUI. Older versions of Raspbian would boot to the command line and the raspi-config tool would run on first boot after installation. Newer versions boot to the GUI and do not run raspi-config. There is a similar GUI configuration tool that can be run from the menus.Although it is already installed on Raspbian, and there is an update option within the menus, it is installed or updated from package raspi-config.

Source: RPi raspi-config – eLinux.org