You can reconnect Dext through the Integrations page. Here's how:
Go to Integrations, located in the upper right of your QBO dashboard.
Select Manage Integrations.
Find Dext there and see if there's a reconnect option.
If you don’t see a reconnection option or if it shows a connection error, you need to go into your Dext account settings, find the QuickBooks Online integration, disconnect it, and then reconnect by re-authorizing with your QBO credentials. Dext will prompt you to log in to QBO and grant access again.