Hi there lisawood,
It's important that all your transactions are correctly entered for your books to be well balanced. QuickBooks Online is a great tool able to help you keep track of sales and expenses so you can see the profit of your company. I can provide some information about merging accounts.
As stated under the Merge duplicate accounts subtitle of this article, there isn't a way to undo merging accounts. You would need to recreate the account by adding it once again to the chart of accounts, then move all transactions manually to the new account.
If you have any other questions, feel free to reach out here.
