A user can upgrade the subscription from the Account Management page of the upgrade subscription of the alexa.amazon.com main setup page. You have to follow all the prompts within the screen to complete the activation process. A user has to keep one thing in mind that if he is downgrading the subscription of his Alexa, he will lose access to all the features contained in the higher plan of the subscription. You can find the list of details included in every plan from the Change Subscription option.