I wrongly connected our VISA with Shopify Holdings and wrongly imported the transactions. Can I undo this?

Hello cindy170,

Welcome to the Community. I'll be happy to provide the info you need so you can continue to use QuickBooks with confidence. 

The program is designed to help you streamline your work using the powerful Banking module. You can disconnect a bank account by navigating to your Transactions page > click on the bank in question > select the pencil ✎ icon >  Edit account info  > hit the Disconnect this account on save checkbox. From here you can click Save so your changes are saved and that the bank is disconnected. Here's a helpful guide that shows you how to: Disconnect accounts connected to online banking in QuickBooks Online. 

Give this a try and let me know how you make out. I'll be a message away in case you need additional support. Otherwise, I wish you a great rest of the day!