You can downgrade your membership to Pro or Basic Account Settings / Billing. Select Downgrade, then you will see the downgrade options on the Pricing page.
You can upgrade to be a Premium member from the Pricing page.
You can find your tax invoices / receipts by heading to Account Settings / Billing. You can download or print your receipts.
You can. You are able to cancel at any time, and the next scheduled payment will not be taken. You will not be refunded for any time remaining on the membership.