I just installed loki 0.4 and was surprised to find the update manager missing. No more easy way to select package sources, drivers etc. Is that a bug or a design choice??
You can install manually that program with this :
sudo apt-get install software-properties-gtk sudo apt-get install update-manager
Updates can be found and handled in AppCenter.
Packages sources modifications are difficult by default for security, the #1 cause of issues last release was PPAs, followed by hardware.