To change your membership type please login at -

Click on the "My Subscription" on the left hand side. You should see something similar to the screenshot below:

When viewing your active membership, you should see a button to change your Membership Type (in red circle above).

Depending on your membership category you will be presented with a list of options similar to the screenshot below: 

At this point make your selection of the appropriate new Membership Type by selecting the checkbox. 

Then click on the 'Purchase' button then go through the check-out procedure to complete the purchase.