In Muon Package Manager :menuselection:`Settings --> Configure Software Sources`
In Muon Package Manager :menuselection:`Settings --> Configure Software Sources`. Directly from the menu you can :menuselection:`Prefrences --> Software Sources`.
In Discover Software Center :menuselection:`Settings --> Ubuntu (Default) --> Settings (Gear icon) --> Software Sources`.
In Discover Software Center :menuselection:`Settings --> Ubuntu (Default) --> Settings (Gear icon) --> Software Sources`.