Thanks for taking the time to reached out to us. Allow me to address your concern, so let me get this straightened out.
QuickBooks Online doesn't have an option to add/change unit of measurement for inventory item. I can see how this feature would be beneficial to your business, I encourage you to send feedback directly to our Product Developer Team. They place a lot of value in the ideas provided by users, when deciding on how best to improve QuickBooks.
Here's how you can send feedback:
Click the Gear Icon.
Click Next and choose Feedback under Category option.
Re: How do i add/change unit of measurement on QB online?
Good day, Hkoi.
For now, we're unable to provide a predicted time period. Rest assured, I will share your concern to our management team so they can submit this to our product developers. They will review all your requests and might include it in future updates.
Visit the Community anytime if you have other concerns.