IInventoryService is what you need, you can get the stocks of a variant by its code, update the data, then save it back 

https://world.episerver.com/documentation/Class-library/?documentId=commerce/8/95A5598E 

Hi Quan,

Thx, could you elaborate a little? ;)

Perhaps with example or reference to a concrete example.
I would much appreciate this.

Here you go: https://world.episerver.com/documentation/Items/Developers-Guide/Episerver-Commerce/9/Warehouses-and-inventories/Warehouses-and-inventories-examples/ 

;) Thx, I've just googlet that...

Hi again,
When I'm trying to  update values in the InventoryRecord, it isn't stored in the database.

What am I doing wrong?

public someClass
{
...

private bool UpdateVariant(VariantData vd)
        {
            if(vd == null)
            {
                return false;
            }
            try
            {

                IEnumerable<InventoryRecord> irs = Helpers.Utilities.QueryInventoriesByEntryCode(vd.Code);
                var editableVariant = irs.First();
                if (editableVariant != null && vd.Quantity > -1)
                {
                    var tmpV = editableVariant.CreateWritableClone();
                    tmpV.PreorderAvailableQuantity = vd.Quantity;
                    var inventoryService = ServiceLocator.Current.GetInstance<IInventoryService>();

                    UpdateInventoryRecord(tmpV);

                    return true;
                }
            }
            catch (Exception)
            {

            }
            return false;
        }
        private void UpdateInventoryRecord(InventoryRecord record)
        {
            var inventoryService = ServiceLocator.Current.GetInstance<IInventoryService>();
            inventoryService.Update(new[] { record });
        }
...

}

What do you mean "it isn't stored in the database." ?

the changes isn't saved to the inventory

Did you make sure inventoryService.Update was actually called, and not the empty catch that swallows any exception? 

No there's no exceptions, it runs the update method on all the WritableClones of the InventoryRecords
Should I perhaps use inventoryservice Save method instead?
Or is it overall a wrong approach. Should I update the 'containing' Variant also or what?
I'm in the dark here...

... When I'm debugging I can see that the InventoryRecord is Readonly (InventoryRecord.IsReadOnly == True).
That might be the issue ;) ...
I'll repport back with a fix/status

OMG!
I'm sorry for wasting your time...
I tried to set the WRONG property...

the right way is to set InventoryRecord.PurchaseAvailableQuantity

...

tmpV.PurchaseAvailableQuantity = (decimal)vd.Quantity;

...

And with use of update method everything works - THX anyway