If you purchased a Stryd Membership on stryd.com/powercenter, you can modify your plan at any time (if you used the Stryd app on your mobile device, you must use the Google or Apple app store to manage your subscription).
Open https://www.stryd.com/powercenter/settings/connect and select "Manage Billing":
Select "Update Plan":
You can select between Monthly, 4-Month and Annual
If you have time remaining on your current subscription, you will get a credit towards your future Stryd Membership (note: if you had a free or discounted membership, your credit is very small or non-existent).