In case you subscribed to Seeking Alpha Premium via Google Play Store, you will need to cancel the monthly subscription first and, once it expires, sign up anew for the annual plan.
To cancel your current monthly subscription, please follow these steps - https://support.google.com/googleplay/answer/7018481?co=GENIE.Platform%3DAndroid&hl=en
You can activate the annual plan either via Google Play or from the website directly. The link for the website is given below.
Customer support service by UserEcho