How do I change my accounting period/year end? I will be having a 18 months accounting period for the first year of change.

Let me guide you on how to change your accounting period, florence_tang.

QuickBooks Online (QBO) Fiscal Year is configured using the first month alone. We can go to the Accounting page to change your accounting period. Then, specify any month as the start of your financial year (also called your financial reporting year or accounting year) for reporting.

Here's how:

1. Click the Gear icon on the top menu.
2. Choose Account and Settings.
3. Tap the Advanced tab.
4. Go to the Accounting section to modify it.
5. Select the appropriate month from the First month of fiscal year drop-down. 
6. Choose either the Same as financial year or January in the First month of tax year field. You can contact your accountant if you need further assistance with this one. 
7. Hit Save.
8. Press Done to exit.

I've got a link here that provides you with articles in managing your business: https://quickbooks.intuit.com/learn-support/help-articles/misc/03/help-articles-global?label=QuickBooks%20Online.

Feel free to reply to this post if you need a hand with reconciling your bank registers or any QBO related. I'll be here to help. Have a great day ahead.