You can change, destroy a will

I am toying with the idea of writing a will specifying how my investments in real estate should be distributed among my nuclear family upon my death. However, I keep changing my mind on how to divide the portions as some of my children are more educated and earning more than others. My question is whether wills are cast in stone once written and whether there are precedence (incidences in law whereby wills have been either cancelled or revoked) and how courts ruled. I want to have a clear mind before I put pen to paper as I am suffering from a terminal ailment that is fast impacting negatively on my health.