To upgrade your plan go to the Account Overview section of your account and select the Change Plan option. When upgrading your plan you will be presented with the cost for upgrading your plan.
If you upgrade your plan mid-billing period you will pay the prorated amount today and pay the new subscription amount on your next billing period.
If you upgrade your plan during the free trial period you will pay the new subscription amount once your free trial period is over.
When you upgrade from free plan to a paid one, you will not receive a free trial period and instead you will be charged the subscription price at the time of the upgrade.