1) According to Bohr electrons revolve around nucleus in definite paths or orbits, so the exact position of the electron at various times will be known to us.
2) In order to explain Bohr's postulate we have to know the velocity and exact position of electron.
3) In order to find the position of electron we have to take the help of suitable light to find the position. As the electrons are very small light of very short wavelength is required for this task.
4) This short wavelength light interacts with the electron and disturbs the motion of the electron.
5) Hence, it is not possible to find the exact position and velocity of electron simultaneously. This was stated by Heisenberg which is called Heisenberg's principle or uncertainty.
6) In this way Heisenberg contradicts Neils Bohr.