If you want to change the tariff plan, you need to write a request in your account. You can do so by following these steps:
Personal Web Panel:
Log in to your account
On left panel click "Requests"
Click "+ New request" in right upper corner
Choose "Other"
Write your request in free form
Mobile App:
Click three lines on left upper corner
Choose "Help"
Choose "Requests"
In the right upper corner click message icon
Select "In free form"
Write your request in free form