I have already posted my entries and need to split some of them into various expense accounts. How do I go about doing this now?

Welcome and thanks for dropping by the Community, @joankmann.

Since the entries were already posted, you can edit them in the register to make some changes. The steps to do this are pretty quick and easy. Here's how:

1. In QuickBooks Online (QBO), go to the Accounting menu at the left pane to get to the Chart of Accounts.
2. Find the account for the transaction, then click View register under the Action column. 
3. Select the specific entry, then select Edit.
4. Make the necessary changes.
5. Hit Save to complete. The transaction now displays Split in the account column.

If you want to learn more about entering split transactions in QBO, check out this article: Enter a split transaction into the register.

You can also add transactions directly to the register. Refer to this article for more information: Add transactions to account registers in QuickBooks Online.

Reach out to me in the comment section below if you have any additional questions. I'll be here to help. Have a good one!