Sorry, but it is not possible to merge or delete multiple accounts. You need to choose one to continue going forward. Most likely you would want to use the one you used for 2018, so you can transfer the 2018 data into the 2019 return next year.
This year did you use a paid edition in one account, but have locked returns in another account that you wish to access? If so, we can tell you how to phone TurboTax Support to tell them you used a paid product and ask them to unlock the returns in the other account.
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/replies/3300041