Absolutely, you can.
Just head to the Plans page, choose an annual plan, click the 'Subscribe' button, and confirm your switch.
However, if you switch to an annual plan within the same subscription level, it will take effect on the next billing date.
Switching to a higher plan (e.g. from Starter Monthly to Pro Annual) is effective immediately.
If you're curious about the precise date when plan changes are processed, you can find more information here.
△ Plans page
△ Page of annual plan subscription