How do you switch AppleCare from monthly to annual?

I don't think there's an annual option for most things. For some items there appear to be monthly or 2 year plans.

If you want to change from a monthly to a single payment plan you had best contact Apple and ask if this is possible.

To contact Apple, click here --> Choose your country or region - Official Apple Support

Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support