If you find you no longer need the plan you’re on and would like to downgrade, you can do so at any time.
Keep in mind, however, that you will not receive a refund or credit for the unused portion of your billing cycle. So, it may be best to wait until the day before your billing cycle renews to downgrade so you’ll have full use of the higher plan level.
Then, once you downgrade you’ll be charged at the lower plan price.