Billing is always done at the start of the month. If you signed up on the 27th of this month, you will be charged again on the 1st of the next month. This is just how Patreon works.
You can find more info on yearly billing on the post on Annual Membership
If you cancel your subscription, you will lose access immediately. You will not have access to content until the end of the month. This is not in my control and that is just how Patreon works.
If you downgrade your pledge, for example from $15 to $10, you will have access to the higher tier until the end of the month. On the next month, you will be billed the lower amount and you will then be at that lower tier.
If you decide to upgrade your pledge, like from $10 to 15, you will be billed the difference (this case $5, taxes might apply) and you will have access to the new tier immediately.
You can switch to an annual membership to save more money. When you switch to annual membership, you’ll get a discount of 15% (about 2 months free). In addition to reducing transaction fees, you’ll help me create quality content faster and reduce the chance your credit card declines in the future.