First make sure that you are signed into the Pandora account that you want to renew.
Then, to renew your Pandora One Subscription, click here.
There is also an auto-renew checkbox available in the Billing area here.
You can find the Expiration date of your subscription here.
If you check that box, the renewal will happen automatically.
If you purchased your subscription through Paypal click here for instructions on managing your Paypal billing preferences.
For information on changing your credit card information, check here.