The user whose cashier stock will be changed must definitely close their cash register with CASHIER SHIFT CLOSE before this process.
After the register is closed go to the Configuration =>Cashiering =>Cashiers menu.
Concerned person is found on the cashiers list and a new cash stock amount is entered to the STARTING AMOUNT section. After this process the user can start doing cashier actions again.
PS: The cashier will not be able to change if you enter this section without closing the register because it is inactive.