Leftovers for a single item can be edited on the product page “Leftovers and product reservation”. Read about it in detail in Leftovers and product reservation.
You need to have option “Leftovers editing” turned on in settings to be able to edit leftovers.
You can look through leftovers as well in general item list, inside one group, in terms of one shop, and etc. - depending on item list filter configuration. You can do it on page Activity > Warehouse > Leftover stock.
If you want to be able to edit leftovers, check if your user is in the group, where leftovers editing rule box is ticked.
Then you will see button “Leftover stock editing” on the page Activity > Warehouse > Leftover stock Pressing it you will be able to edit leftover values of each product offering in each warehouse.