Switch off basic account at next month

how to switch off automatic payments for my account and the next month my account will free?

@smival - Just go to the Account page and click the Downgrade link under Free membership in the membership chart.

after that “Free membership” become active at the expiration of “Basic membership” or immediately?

Downgrading to Free changes your membership
level immediately, so you might want to downgrade the day before your next billing date.