Welcome to the community!
Generally, if you've made extra payments on your account, the excess amount would be credited on your account. It would appear as a negative balance on your My Account. The credited amount would be used toward your subsequent bill.
If you wished, you could request to be reimbursed the amount. To do so, you would need to contact customer service. They can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above. Depending on your payment method, it may take a little while for the reimbursement to be processed, though.
Hope this helps 😀