I have posted transactions on QuickBooks and also syncronised with Bank; I subsequently posted some of the transactions.

Hello, userchristi. 

Thank you for reaching out to the Community. Let me walk you through deleting your QuickBooks Online data and start a new one. 

You can purge your data if your account is less than 60 days. Please keep in mind that this method only works for QuickBooks Online Plus and Essentials. This process will delete all data in the company file. Once you delete, you can’t restore it. 

1. Sign in to your QuickBooks Online Company.
2. Change the URL to include /purgecompany
   Example: Change https://sg.qbo.intuit.com/app/homepage to https://sg.qbo.intuit.com/app/purgecompany
3. The next screen will provide a summary of the items to delete. 
4. Type YES to delete the data, then select OK. 

Reminders: 

• If the industry you want to use is different than the one showing, feel free to change it.
• Mark whether you would like the system to create accounts based on your industry or start with an empty chart of accounts.
• Once your selections are complete, select Wipe Data. When the deletion is complete, you will be rerouted to your homepage. 

You can read through this article for additional information: Delete your data and start over in QuickBooks Online. 

Please don't hesitate to leave a comment if you have further concerns about resetting QuickBooks. I'm just around if you need help. Stay safe!