Instead of purchasing an annual subscription through the app directly, you can save 8% by buying your subscription through the 'Manage Account' link on the Settings page of the app, or through the web. Just create an account through the app (or log into one if you have already created an account), then use the new 'Manage Account' link (you'll need to update the app to the latest version to see this link in the Settings page). As long as you are logged into the same UAVForecast account on your mobile devices and on the web, one subscription will work across all your devices and the web.
Please note that if you want to switch from monthly to annual, you cancel your subscription using the link on the Account page. Your subscription will continue to be active through the last day of the billing period you have already paid for, but you won't be charged the recurring $1.99 monthly payment again. Then, once the original billing period has ended, you can just click the 'Subscribe Now - Yearly Subscription' button to purchase a new annual subscription with the 8% savings. Note that this is entirely optional and if you want to continue being a monthly subscriber, you absolutely can continue to do so. Both the monthly and annual subscription options are available for new subscribers, too.
Also, please note that Apple and Google Play do not permit developers to advertise alternate payment methods and discounts like this within the app itself.