Thanks, nadebula.1984,
Your question led me to investigate Magic Banana's answer further. The thing is, that 'https://packages.riot.im/debian' doesn't show up in the 'Other software' tab under 'Software and updates' (in Mate's control center. But I realised that I can make the change from 'Flidas' to 'Xenial' in the etc/apt/sources.list.d/riot-list.
