Yes it is possible to upgrade or downgrade your account. You can upgrade immediately, downgrading will be processed after the end of the current package period.
Managing your plan is easy by following these steps:
Step 1: Login to your account
Step 2: Click on the first letter of your name in the top right corner - and than on ‘Account
Step 3: Click on the section ‘Subscription’
Step 4: Choose the right plan, and activate it.