You can upgrade your subscription within the application.
- Click on your account on the right hand of your screen,
- Then click on Account settings ,
- Select Billing History link, then select Change Plan
- Select the plan you want to upgrade to
You can change your payment method at this stage if you like.
You can see your subscription status at Plans summary tab. You can get invoice and Receipt from Billing History section at the bottom. Also you will get a confirmation email about your transaction.