How to delete an unwanted account

I have an account with a pimary and secondary email adresses set up for some time. I was trying to make the secondary email address into the primary address and the old primary into the secondary .  Somehow I appear to have created a second account with the old secondary as the new primary. I do not need two accounts and the cost of a second account. What do I need to do to delete one of them? (As a note - when I look at the history on each of the accounts, it shows me the same history for the original acccount on both.) Any suggestions appreciated - Thankyou.