You can update your billing method by clicking on the round icon in the upper right corner of your account, going to Settings > Billing Methods. You can use either PayPal account or a credit card as a billing method.
Please, note that on hourly contracts you are billed automatically according to the Weekly Billing Cycle. You are able to access your freelancer's Work Diary in real time throughout the work week and have an option to dispute the hours you are billed for during the review period.
Please, check this help article for more information.