The water tank is just above the first floor of the house. The diameter of pipe from the tank is 25mm and diameter inside the bathroom is 12 mm. Water pressure in bathroom of ground floor is good and the water pressure in the bathroom of first floor is poor. The tank is inside the roof so it cannot be raised. The bathroom has internal piping system so it cannot be removed without breaking the tiles. In such circumstances, if I increase the outside pipe diameter to 50 mm, will it help to increase the pressure inside the bathrooms.
Is there some other solutions?
Increasing the pipe size from 25mm to 50mm will definitely reduce the pressure loss along the pipeline, making more pressure available; however, 25mm is already a pretty good size for a residential water pipe and the pressure loss may already be very small. So the difference may not be noticeable. Unfortunately, the problem is likely just the lack of vertical head from the tank to the bathroom.
In some places they make tanks with a bladder or spring inside which are pressurized as they are filled to provide more pressure than gravity alone can.