You can reduce the quantity in stock of a kit, and at the same time increase the quantity in stock of the individual parts that make up the kit using the Break Out Kit function.
To Break Out a Part Kit:
Open Inventory > Parts

When the Inventory Parts window opens, locate the Kit you wish to break out into individual parts.
Double click the Kit and it will open Part/Kit window and display your warehouses. Right click on the warehouse that the kit is in. You will see a window with options and select Break Out Kit.

When you select Break Out Kit a new window will pop up giving you the warehouse, the break out date, and quantity to break out. You will need to enter the amount you wish to break out before you save.