Either through the GUI package manager, Discover. There should be a button to update all your software.
If you would want to do it via the terminal. Type the command:
sudo pkcon refresh && sudo pkcon update
This would update all packages. For KDE Neon, it is generally recommended to use pkcon (package manager) to update and apt (package manager for Debian/Ubuntu based distributions) solely for installing packages. Typically APT could be used to install and update/upgrade packages, but for KDE Neon, due to using the latest kde software compared to other Ubuntu based distros, it is suggested to use PKCON.