On the iPhone app:
Go to your collection page
Click into any wine
Scroll down to "Custom Details"
Tap the arrow beside the title
Select the trash bin icon next to the field you would like to remove
On the web app:
Go to your collection https://app.invintory.com/collection
Click into any wine
Scroll down on the label details page to "Custom Fields"
Tap the button "Edit" beside the title
Tap the trash bin icon of the field you would like to delete