How to change the previous period transactions?

Hello, qber-eternalrobo. 

I'll give some details regarding the error message, and help you change and save a previous entry. 

This happens when you tried editing a transaction when the "Close the books" setting is active. Since you need to edit an entry, simply click Yes on the message prompt to save the changes.

However if it prompts you to enter a password enter it on the box, then proceed with the edit. 

If you don't use the "Close the books" feature, you can turn it off from the Account and settings page: 

1. Click the Gear icon, then select Account and settings.
2. Go to the Advanced tab.
3. Expand the Accounting section.
4. Toggle the Close the books option to off.
5. Click Save, then Done. 

If you don't know the password or if you can't turn it off, you'll want to contact your company administrator. 

As what the message suggests, editing a transaction will affect your books. I'd recommend consulting an accountant before doing this. They'll ensure the edits and the reporting are correct. 

More details about the setting can be found here: Edit your closed books.

Need to check your financials through reports? I'm sure you'll find this article helpful: Run reports in QuickBooks Online. 

If you have more questions you'd like me to address, please let me know. I'm here to help you record your transactions, check your settings and more.