Unfortunately, the only way to change an inventory part to a non-inventory part in QuickBooks is to create a new item in your item list and classify it as a non-inventory part.
As you can see from the screen shot below, there is no option to reset the type from inventory part to non-inventory part:
Even worse - once you create the new non-inventory part, you can't use the exact same name as it currently appears. You'll have to make it unique by one character (i.e. add a * at the end of the part number or something similar) so this new non-inventory part can be saved in your item list.
MOVING FROM NON-INVENTORY PART TO INVENTORY PART
Good news here!
If you want to convert a non-inventory part to an inventory part, you can do that directly within the item itself by clicking the drop down and choosing "inventory part".
Before you save your change, QuickBooks will alert you with a warning message indicating this change cannot be reversed, as shown below:
ALWAYS MAKE A BACKUP BEFORE MAKING CHANGES
It's good practice to make a backup copy of your QuickBooks data prior to making any significant changes such as these. That way, you can easily rewind to your backup copy should you change your mind or if something doesn't work out the way you intended.