Start off by pouring some white vinegar on the nut and scrubbing it with a wire brush. This will help you to loosen and eventually remove any mineral deposits that you notice on the nut that is stubbornly refusing to rotate. After you have loosened the mineral deposits, clean the area by rinsing it and leaving it alone for a few minutes to dry.
Now grab a wrench and try rotating the nut to loosen it. Do not forget that turning the nut clockwise will tighten it, while turning it counter-clockwise will loosen it.
If the nut does not come free easily, try turning the wrench in the opposite direction, i.e clockwise. Do not worry about tightening the nut. While this will come off as a bit paradoxical to you, understand that if you are successfully able to turn the nut at all in any direction, even clockwise, it will loosen up and consequently make it possible for you to turn it in the counter-clockwise direction.
If the nut still does not turn, get a centre punch and hammer. Place the centre punch onto the nut and tap it with the hammer. Do not strike the centre punch with too much force as that may result in causing damage to the surrounding plumping. After a few gentle strikes, try loosening the nut with the wrench again.
If you were unable to move the nut even after using a centre punch and hammer, you should try using some science to your advantage. You may remember learning in school that heat causes metals to expand. If you can manage to make the nut expand, you may be able to turn and remove the nut easily. If you have a hair dryer at home, plug it in and hold it close to the nut to heat it. After heating the nut for a minute or two, try turning it with the wrench.
Try loosening the nut using lubricating oil. Squeeze the oil onto the nut and give it a few hours to set. Do this again a few times over the course of the day and finally try turning the nut with the wrench. Hopefully it should turn without any problem now.