Can you make a "mass update" on accounts? I accidentally coded a bunch of small transactions to a wrong expense category & I'd like to fix them all at once.

Welcome to the Community, @cschreng.

I'll be happy to explain how to do a mass update on your accounts. 

One way you can correct your accounts in one go would be with a Journal Entry. 

In traditional accounting, a journal entry is a transaction with at least two parts - a debit and a credit.

 As a rule, the Debit column total should be equal to the total of the Credit column.

When you record a transaction with a journal entry, QuickBooks Online labels the transaction as Journal in the register or account history, and Journal Entry on reports that list transactions.

Use journal entries to:

• Transfer money between income and expense accounts.

• Transfer money from an asset, liability, or equity account to an income or expense account.

• Use the traditional system of accounting that enters debits and credits in a general journal or ledger.

To create a Journal Entry:

1. Select the Plus icon (+) on the Toolbar.

2. Under Other, select Journal Entry.

3. Enter the transaction date in the Journal date field.

4. (Optional)

   Enter a Journal number to help identify the transaction in reports.

5. On the first distribution line, in the Accounts field, enter any account listed in the Chart of Accounts.

   
   Note: If you are not sure which account receives the debit and the credit side of a transaction, consult your accounting professional for guidance.
6.  

7. Enter the transaction amount in the Debits or Credits column.

    

   Tip: The first line is typically a Debit. Once you save the transaction, you can find it in the Recent Transactions report to see whether the Debit entry had the desired effect.

8. (Optional)

   Enter a Description for the transaction.

   
   The Description you enter appears on reports that display transaction details.

9. Specify a customer, vendor/vendor, or employee associated with the line, if any, in the Name field.

    

   Note: QuickBooks Online Plus users – When the Classes and Locations feature is turned on, additional fields for Location and Class appear.

10. Continue to enter distribution lines until the sum of the Debit column entries equals the sum of the Credit column entries.

     

    Note: QuickBooks Online Plus users – When the Classes and Locations feature is turned on, additional fields for Location and Class appear.

11. (Optional)

    Select Make recurring to set a recurring journal entry schedule.

12. Select Save.

For more information on Journal Entries, click here.

Please keep me posted on your results with this here, the Community has you covered. Thanks again, I'll be standing by for your response.