You could open up your belly button by cutting the belly button however you really should not do that because if you do open up your belly button it can cause a serious problem.
By serious I mean you would likely bleed and need medical attention right away to close it back up before you died.
The umbilical cord attaches to your insides where it provided nutrients, vitamins etc from your mother to you when you were in the womb.
The umbilical cord that was cut off where your belly button is is basically a feeding tube for the baby and when it is cut off then it dries off and then closes up the hold in your belly.
So cutting the belly button open would be a really bad idea.