That's the problem when you try to update inventory records, they are strongly linked.
For example, the amounts in the InventSum table must ALWAYS reflect the records on the InventTrans table and the InventTrans table quantities must ALWAYS reflect the quantities on the source record (Purchase Order, Production Order and etc.).
To force a change on the InventSum you first need to understand which InventTrans records were used to compose that quantity and if you decide to update the InventSum anyway, then you will have to update the InventTrans as well, to keep the quantities balanced. But if you update the InventTrans, they may stop reflecting their source record. Do you see where I'm going?
Forget about hard updates for a minute and think about a movement journal or a count journal where you could change inventory quantities without messing with the system integrity.
NOTE: If you still need to update InventSum records, you will have to look into those display methods to understand what is behind them and what fields (and tables) you really need to update.
