You can up- or downgrade your membership easily in your account settings. Here's how it's done:

1. Log in to your Subba-profile
2. Head over to My Account 
3. Click the 'up- or downgrade' button
4. Select the membership you would like

You will switch automatically to the new membership on the end-date of your current membership.

Heads up: Up- or downgrading is not possible when your payment is being processed.