When spontaneous processes are accompanied with an increase in entropy of universe and remains unchanged in a reversible process, what happens to entropy when one process is non spontaneous? Would entropy decrease??
Yes. That is why non-spontaneous reactions do not occur unless you provide some artificial interference. If you were to cause a non-spontaneous reaction to occur in this way, in that single reaction, entropy would decrease.
However, in doing so, the entropy of the universe will still increase or stay the same BECAUSE, in the process of creating such an artificial interference, entropy is created.
For example, if you add energy for a reaction to occur, the energy must be obtained from... well, a non-spontaneous process, where an increase in entropy occurs.
Or more simplistically, if your room is very messy (high entropy), and you clean your room (lower entropy), it may seem like the entropy has just decreased. Actually, the entropy has still increased because in order to clean your room, you do work, which involves multiple other increases in entropy: digesting food which came from a food-processing plant, which was driven to this plant by a truck that adds entropy by burning gasoline, and so on.
Start by remembering that when we say that a process is spontaneous we mean under certain conditions e.g. at room temperature. The reverse of spontaneous processes are non-spontaneous processes. Non-spontaneous processes do not occur naturally according to classical thermodynamics and it is statistically extremely improbable that they will occur naturally according to Boltzmann thermodynamics.
But non-spontaneous processes can be forced to occur with the continuous expenditure of energy. For example, the expansion of a gas in a piston can be reversed using a heavy weight on the piston. A chemical example of a non-spontaneous process is provided by the electrolysis of water. In both these cases, the values of the thermodynamic parameters for the non-spontaneous process are equal to those for the spontaneous process but of opposite sign. However, this ignores the source of the consequence of using the energy we use to bring these processes about. As Frank put it (above), 'the process of creating such an artificial interference, entropy is created'. For example, in the electrolysis of water, the entropy change of the H2/O2/H2O system is negative, but because the use of electricity causes the water being electrolysed to warm up, this involves thermal enerfgy transfer to its surroundings, causing a positive entropy change and the overall entropy of the universe increases. Spontaneous or non-spontaneous, the entropy of the universe always increases.