If you want to change your free package to Power package then tap on the balance bubble and choose Me-> Subscription-> Upgrade to Power. Once you confirm the action – the monthly fee, according to your age, will be charged. You can read more about prices from here. The monthly fee will be charged each month on the same date as you chose the package.
You can return to the free package at all times, for that you have to choose Me-> Subscription-> Downgrade to Basic. Basic package will activate after the paid period ends. In case you also had a card, then it will automatically close with the Basic package activation.
For example: your payment date is on the 1st of January, on the 15th of January you chose the Basic package, therefore on the 1st of February the Basic package will activate and your card will be automatically closed. If you wish to continue using your card and the Power package then you can cancel the package change before 1st of February.