To switch the current to a higher plan, you can go to your account page, click the Billing button and upgrade it manually. Or you can press your profile photo, and click the "Upgrade" button.
The number of days not used in your current plan can be deducted by the corresponding amount.
However, it's not able to downgrade a subscription at present. Instead, simply cancel your subscription first, and after the current plan ends, re-upgrade in order to make this sort of change.