Can I switch my plan from monthly to yearly billing?

Not for the plan you are already on. On the subscription page your current plan is marked Active and can't be selected, so the admin can't change only its billing cycle. Contact Wcart support and they can help you move it to yearly billing.

You can choose yearly billing when you move to a different plan:

1. Go to Settings → Account → Subscription.
2. Under Upgrade / change plan, click Yearly in the cycle switch. Monthly and Yearly always show. Quarterly and Half-yearly appear only when a plan is priced on them.
3. Click the plan you want.
4. Check the Payment Summary. It shows what you pay now and how the change is billed.
5. Click Proceed to payment, or Confirm change if nothing is charged now.