If you created a Sling TV account through your Amazon Fire TV and Fire tablet, you can add or remove DVR to your subscription in the Sling TV app.
Launch the Sling TV app, then scroll to the right and choose Settings. Then visit your Account.
Here you can change your service by clicking Add/Remove Service.
After clicking Add/Remove Service, click Continue to move onto the Extras page.
Select Cloud DVR to add or remove the Cloud DVR feature to your subscription.
Next, you will see an updated review of your service. Be sure to review your order before pressing FINISH & SUBMIT.
If you added DVR, choose Start 1 week free trial.
You will be presented with a summary of your subscription. Then choose Close.