Currently, you cannot upgrade or downgrade on your own.
There are two ways to change the plan
In the App
Go to the account page
If you are paid user you will see a card "Your Plan Details"
Click "Change Plan"

Card "Your Plan Details" from account page
Dialog will open (see screenshot)
Tell us what you want us to do for you in the form of a message

Dialog for upgrade or downgrade plan
We will process your request within 24 hours.
Chat
Click on the help button on right-hand side
Tell us to upgrade or downgrade your plan
Depending on our availability in the chat we will do it immediately or in the next working day