You may change your streaming plan at any time by visiting your account page! It's quite simple:
1. Visit your account page.
2. Select the Change Plan link within the Plan Details section of the page.
3. Choose the plan that you would like to change to. You will automatically be switched to the new streaming resolution (4k <-> standard) or billing period (monthly <-> annual).
4. Your card will automatically be charged at the start of your next billing period in the amount of your new plan!