How to change email templates in system > email ?

Hi,

Telegram ? Are you talking about that:
en.wikipedia.org/wiki/Telegram_(software)
In that case, it would require a specific plugin to be developed in order to integrate with the telegram API. Modifying the emails won't help as far as I can see.

The HTML version is the HTML template that is used to generate the content of the email for email clients supporting HTML.
The text version is the version of the email sent for email clients not supporting HTML
And the preload version is loading the necessary data for the HTML version so that it can replace the tags in the HTML version by the corresponding data.