Welcome to the Community, @accounting-think. I'm here to assist you in sorting out this issue.
Beforehand, I'd like to ask for some information to help me get on the same page. Are you still in the setup process when the address defaults to Singapore? Additional information would be much appreciated.
If it's done, please know that we're unable to modify the country. However, you can cancel it and recreate a new one.
Once everything is all set, here's a great resource that can guide you further QuickBooks Online: Get started with QuickBooks Online.
Please don't hesitate to tag my name in the comment section below if you have other questions or concerns about managing your company information. I'm here to help.
