Go to the ground floor and get out of the building from the right door of the building (the door opposite to Georgy green). On the right side just a few meters ahead of the main door, there is another door, usually it is closed, and nitrogen bottles just outside that door and protected by a metal grid and a lock. Knock that door, ask any staff inside for the key to open the grid, and borrow a trolley. Using the trolley, move the empty bottle to where nitrogen is and change for a full bottle labeled with a white cap, sign the record book (you need to know which project to pay for the gas, and the project code). After putting the full bottle to where as needed, you should return the trolley to the store.