Billing date

Hi. I need help changing my billing responsibility date from the 1st of the month to the 15th of the month. I get paid w times a month on the 15th and 30th of eacH month my first pay covers my bills and second pay is for the rent and because I have things set up as such I am being hit for a late fee every month and I'm sick of it. How do I change the billing responsibility date from the 1st to the 15th so I can stop getting hit with late fees. I understand there will be proration of my bill and what not just need it done. Please help at&t