H2O is a weak field ligand and it will have low value of ∆0. As a result, the electrons can fill lower set of dorbitals as well as upper set as : t42ge2g
On the other hand, CN– is strong field ligand, so that ∆0. will be high. As a result, electron will try to remain only in the lower set and will therefore pair up i.e., t62g.
