It is straight-forward to adjust a stock quantity in FLEX. Open up the item and double click, as if to update.

Right click (like when you went to Add a Similar Item) and press Adjust Quantity >

A drop-down menu will appear and the relevant quantity adjustment can be actioned.

Again, this may ask for a reference number, and will ask for the new amount in stock.

The movement history will update in the bottom section of the window, showing the date and time of adjustment, along with the reason for the change.