You can refer to Doc ID 626475.1 on https://support.oracle.com for details on changing the decimal precision of quantities with JDE.
This is actually a very important decision that you need to make as part of your manufacturing implementation so you should have some consulting support to walk you through it. For example, going down to 5 decimal places can be painful if you don't use that sort of precision for many items. In the case of the example provided you may be better served going to grams as your primary unit of measure for the affected item.
JDE does not store floating point numbers in the database but rather it will store an integer with a nominated precision. In the case of quantities this is a 15 digit integer. So if you have 5 decimal places then 1KG would be stored as 100000 in the table and the maximum quantity you can store is 10,000,000,000KG.