For data consistency reasons it is not possible to update the quantity of stock lot directly.
If you need to adjust a quantity of a particular stock lot, then it can be done through the document that created that lot:
if this lot is purchased, please open the purchase order, and adjust the quantity there;
if the lot is produced, please open the manufacturing order;
if the lot is added manually, please create another lot or write-off.
Probably the easiest way is the following:
go to Stock -> Stock lots, find the lot and open lot's details;
there is a field "Source" that contains whether a link to the parent document or a text "Manual lot";
if the lot is not created manually, click the link, and adjust the quantity there.