How to install Signal on Ubuntu 21.04 (Hirsute Hippo)
Open Terminal and run this command
Install signal official public software signing key
wget -O- https://updates.signal.org/desktop/apt/keys.asc | gpg –dearmor > signal-desktop-keyring.gpg
sudo mv signal-desktop-keyring.gpg /usr/share/keyrings/
Add signal repository to your list of repositories
echo ‘deb [arch=amd64 signed-by=/usr/share/keyrings/signal-desktop-keyring.gpg] https://updates.signal.org/desktop/apt xenial main’ |\
sudo tee -a /etc/apt/sources.list.d/signal-xenial.list
Update database reposiory and install signal
sudo apt update && sudo apt install signal-desktop