We investigate the accretion induced spin up of the black hole via numerical simulations. Our method is based on general-relativistic hydrodynamics of the slowly-rotating flows in the Kerr metric. We account for the changing black hole mass and spin during accretion. We study non-magnetized flows with shocks, and finally, we also include magnetic field endowed in the gas. The aim of this study is to verify whether the high mass black holes maybe produced with large spins, even though at birth the collapsars might have contained non-spinning,or moderately-spinning cores. In this way, we put constraints on the content of angular momentum in the collapsing massive stars.