I try to correct my billing info on my iPad mini 2,but it declines it.What do I do?

If it's a debit card that might be why, not all seem to still work with iTunes. Do you have another card that you could try ?

For a card to have a chance of being accepted it needs to be registered to the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you are (and therefore the country which is on your iTunes account).