This paper proposes a reliable pairwise key-updating (RPKU) scheme for clustered WSNs via predistribution and local collaboration approaches. Based on the modified version of Blom's matrix construction, this scheme can extend and shrink the pairwise keys in WSNs with the network topology changes. This scheme also presents a hierarchical key distribution method in the clustered WSNs, guaranteeing that any pair of neighboring nodes can find a common secret key between themselves. Comparison and simulation results show that the proposed scheme outperforms most of the existing pairwise key establishment schemes in terms of network security, key connectivity and scalability.