Follow these instructions to Package your Integration, ready for migration between ServiceNow instances.
How to package and export an Integration ready to be imported to another instance:
In Unifi Integration Designer, navigate to and open < Your Integration >.
Click the 'Integration' icon to open the Details page.
Click Package.

The Package Integration modal is displayed.
Confirm by clicking Package.

Unifi packages the Integration into an Update Set which is automatically downloaded.

The Integration Package Worker modal is displayed.
Notice: the modal tells us how many records were processed and the name of the Update Set that was created (the integration name prepended with the date/time).
Copy the name of the Update Set.
Click Done to close the modal.

Navigate to the downloaded Update Set and Show in folder*.
*Example shown in Chrome. Other browsers may vary.
Then rename the file (using your Update Set name in order to easily identify it when uploading to the other instance).

Congratulations! You now have the Update Set containing the complete Integration available to be imported to your other instance.
You can find more information on how to Load customizations from a single XML file in the ServiceNow Product Documentation.