How to remove Telegram?

If you downloaded Telegram directly from the website, then your package management system has no idea about it, and you have to remove the files yourself. You may find them in ~/Downloads but perhaps you put them somewhere else, for example /opt. If you have been using Telegram you may also find an automatically created directory in ~/Downloads called Telegram Desktop which contains files you have downloaded from Telegram messages. There may also be a hidden directory in your home ~/.TelegramDesktop. You can remove Telegram by deleting all these files and directories.

If you can't find them dosudo updatedb && locate Telegram to see where they are.

If you installed from the Atareao PPA then remove the PPA itself:

sudo apt remove telegram
sudo add-apt-repository -r ppa:atareao/telegram

As mentioned by @Anwar, there is a package called ppa-purge which removes ppas more gracefully, attempting to restore any packages from ppa to repository versions as well as removing the ppa versions. You don't need it in this case, since there is no repository version of Telegram, however, it is the preferred way to remove ppas:

sudo apt install ppa-purge
sudo ppa-purge ppa:atareao/telegram

This will uninstall any packages from the ppa as well as removing it.

If you installed the snap package telegram-sergiusens then do

sudo snap remove telegram-sergiusens

search and delete the "TelegramDesktop" folder.

in my case:

lsof|grep Telegram

revealed it was under:

~/.local/share/TelegramDesktop/