Although I understand the current covid-19 situation, your system made impossible to cancel a mobile plan. I've looked thu every possibility in your website and app, and I could not find a way to cancel my mobile plan.
Hence, I would kindly ask for your team assistance to do so. Moreover, it is not my fault you do not make available a reasonable channel for plans changes. The only option is to "upgrade" an existing plan.
The only methods for contacting Telstra to cancel your mobile plan is to either call 132200 or use the messaging feature in the My Telstra app (the blue icon in the bottom right or the "Get Help" screen).
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.