After you have created a SLING account, in addition to the SLING services included in the promotion, you can add Extras, Premiums, additional hours of Cloud DVR and more. You must prepay SLING for any additional services not included in the promotion, including any associated taxes.
From the SLING website:
Sign in to your account on the SLING website and select Change Subscription. Follow the prompts to add, remove, or swap your subscriptions.
From the SLING app:
Launch the SLING app and go to the My TV page. Scroll down until you find the Add Premium Channels & More ribbon. Follow the prompts to add Extras, Premiums, or additional hours of Cloud DVR.
Charges for any Extras added to your SLING subscription are collected immediately by SLING at a prorated amount for the remainder of your current SLING billing cycle. On your SLING renewal date, you'll be charged the full price for your new SLING service.
When removing an Extra from your SLING subscription, the Extra will be removed on your renewal date, and you won't be charged for the SLING service.
For additional information, refer to SLING's website for instructions on adding and removing channels and Extras.