I can show you the easiest way on how to change your existing build assembly, AWBEL.

You can press Ctrl + D or delete the assembly. Then, recreate or build it again. I have attached a screenshot of how you can delete it:

Once done you can assemble your products or start building them again. But, I still recommend reaching out to your accountant's advice for on the proper way on how to do the adjustment so it wouldn't mess up your books. 

For your reference, you may also want to adjust the stocks of your finished goods without affecting your stock of components. 

Feel free to let us know if you have questions about the process. Remember, we're always right here to help you anytime.