Key update is a challenging task in resource-constrained networks where limitations in terms of computation, memory, and energy restrict the proper use of security mechanisms. We present an automated tool that computes the optimal key update strategy for any given resource-constrained network. We developed a push-button solution - powered by stochastic model checking - that network designers can easily benefit from, and it paves the way for consumers to set up key update related security parameters. Key Update Assistant, as we named it, runs necessary model checking operations and determines the optimal key update strategy that satisfies given security and performance requirements.