Strava Premium is our subscription service providing more data for athletes.
If you're a Premium Member on a monthly billing plan you can upgrade to the yearly plan and save 18% on your membership fees. If you're logged in to your account click here and click on the button to "Upgrade to Annual for $59/year." Your new Annual membership will be added on to your existing monthly premium membership - meaning that there won't be any 'overlap' or loss of Premium service.
Alternatively, you can access this from your account by hovering over your name and profile pic and from the drop down menu that appears click on "Account."