To subscribe to an annual plan, go to splice.com/plans and select the plan you want. On the following checkout page, you will see a "choose billing cycle" section, where you can choose the annual version of the plan you selected:
Note: it is not currently possible to switch directly from a monthly to an annual plan. If you're a monthly subscriber looking to switch, please cancel your current subscription, and then wait until the end of the current billing period (your plan will remain active during this time). Once the billing period ends, you can re-subscribe to your preferred annual plan via splice.com/plans.