Step 1: While creating the invoice Click on add products >> Select the product >> Click on the unit changing option.
Step 2: Change the unit of the product >> add the quantity of the product >> Click on continue.
Step 3: Add the other details >> Click on create button.
Note: Please note that to select the alternative unit for the product first you need to add alternative unit for the product in product details
To add alternative unit to the existing product please check the below link.