You can change your monthly billing cycle date by downgrading to a Freelancer Basic plan before your billing date this month and then upgrading back to a Freelancer Plus plan the following month on your preferred billing date. You can't downgrade and then upgrade for the purpose of changing your billing date within the same monthly billing cycle.
For example: If your current billing date for a monthly plan is on November 15, and you want to change it to the 1st, you'd need to downgrade before November 15 and then re-enroll in the Freelancer Plus plan on December 1. If you downgraded after November 15, you'd need to wait until January 1 in order to renew your billing cycle.