How do i import revenue data from another Point of Sales system in to QB online

You need to use an importer tool to import data from POS as the Sales Receipt

https://get.transactionpro.com/qbo

Hi there. I encourage you to check out the following article as it shows you how to: Import Data into QuickBooks Online.  You can also look into using third-party data migration apps that you can search for on our app store by visiting www.apps.com. I recommend choosing an app that suits your business needs based on the app description and the reviews. 

Let me know if this info helps. I'll be one message away in case you have other questions. Otherwise, I wish you a great rest of the week.