If you want to downgrade your current plan, you can choose a new one on the pricing page. It will be activated upon the expiration of your current subscription and you will be charged accordingly.
However, if you subscribed through the Android app, the new plan will be activated as soon as you downgrade. All remaining days of your previous plan will be automatically converted to days of the new plan at an equivalent cost.
Please note, a downgraded plan can be set only for the same billing period. Annual - monthly changes cannot be processed automatically. If you want to downgrade your billing cycle as well, you should turn off the auto renewal of your current annual subscription, wait for it to expire and subscribe monthly after that.
Here you can check how to downgrade a subscription purchased in the mobile app.