The best way to get riot installed is to install it from the official repositories, this way your package manager can keep it up to date for you. All the info you need is on https://riot.im/desktop but along with no instructions, the ones below should work for most users but do assume you have wget, software-properties-common, sudo and the apt wrapper.

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set -e
wget -qO - https://riot.im/packages/debian/repo-key.asc | sudo apt-key add -
sudo apt-add-repository https://riot.im/packages/debian/
sudo apt update
sudo apt install riot-web