I am reducing fat for a month doing gym and spinning, and now I'd want to add cycling to it. On spinning days I've been eating like in non-training days, about 2100kcal. But it lasted 1,5h max, what about long 5h cycling session days? How much should I increase my calories intake? What should I eat?
Well to start with, you don't just leap from 1.5hr spin sessions to 5 hour rides - unless you have a long history of cycling this would most likely not end well.
However, to answer the nutrition question:
While you are riding, there is a limit to how many calories you can absorb. Most sources will quote 60-75g of carbohydrate per hour (240-300kcal per hour). You will want to choose an energy source that is mostly carbohydrate, although a small amount of protein can protect muscle mass and improve recovery.
Over a 5 hour ride this will add up to in the region of 1200-1500 extra calories, which may seem a lot, but will still be less than you are burning.
As you become more experienced with this type of ride, your body will adapt to use fat as its primary fuel source and you will find you are able to gradually reduce your energy intake should you wish to do so.