At any time, you can upgrade from your current plan to a higher plan.
Note that it's not possible to downgrade a plan. If you need to downgrade, you will need to cancel your subscription, then subscribe to the lesser plan when your term expires.
To upgrade, go to "Change/Cancel your Plan" in the menu:
Click "Edit" on the subscription you wish to change.
From here, you can click on the "Change Subscription" button...
Then, select the upgraded plan from the dropdown:
The cost for your upgrade will be prorated depending on how many days are remaining in your cycle. Your credit card will be charged immediately.