We currently don't have an automated way to change a subscription from monthly to yearly. Thankfully the process is very easy. Simply go to http://admin.xogo.io and log in with the same email / password that you use for the XOGO Manager app.
After logging in buy as many annual subscriptions as you need. Next open the XOGO Manager app click on each of your players in turn, and in the dropdown menus select the annual licenses that you purchased. See this video: https://youtu.be/xcstEpLxBIw
Once you are satisfied with the migration - and to avoid being billed for the original monthly subscriptions - go back into http://admin.xogo.io and cancel the monthly subscriptions that you no longer need.