It is unwise to cut open a boil of any kind unless under medical supervision. The chances of getting infected are too high for you to take a risk doing this yourself. It is best to get medical advice on whether you should be cutting open a boil on your own or not. If your boil is extremely painful, it is advisable to go to a doctor and get it lanced. For some immediate relief at home, you can apply a thick paste of turmeric powder and clarified butter to it. This will help to heal it and also will bring down any pain and swelling that may be present. Furthermore, if your boil has reached its own maximum capacity of expansion, this paste will also help to burst it quite painlessly.
If the aim is to release pus from your boil, then you can try to apply a warm pack around it. This will force the boil to burst open and release pus and is also the most harmless way to do this, though it may not be the most painless. You can also apply freshly squeezed juice of onion or garlic to boils. You may have to do this a few times before you release any pus, however.
answered by M W