If pre-order was enabled for the product via the All Products tab
Go to the All Products tab
Uncheck the 'Enable pre-order for all products using Default Settings'
Click Save
If you need to keep 'Enable pre-order for all products using Default Settings' enabled and want to disable pre-order for a single product:
Go to the Single Products tab
Search for your product and click on it to enter the edit settings screen
Leave 'Enable pre-order for this variant' disabled for all variants
Click the Save button
This will create a disabled single product setting that will override the All Products setting for this product.
If pre-order was enabled for the product via the Tags tab
Go to the Tags tab
Find the tag setting responsible for putting the product on pre-order
Delete it
If pre-order was enabled for the product via the Single Products tab
Go to the Single Products tab
Search for the product
In the search results, click the trash can icon beside the product to delete the pre-order settings for that product.
Alternatively, if you'd like to disable but not delete those pre-order settings:
Edit the pre-order settings
Uncheck the box Enable pre-order for this variant
Click the button Copy Settings to All Variants
Click Save