Embedded systems are deeply integrated into critical applications but, despite their importance, lack an effective means to apply over-the-air software patches without significant downtime. Standard mechanisms for firmware updates require device reboots that wipe important in-memory state. Prior efforts have proposed "live" updates to address this problem, applying patches to an embedded application without a reset, but they tackle a limited set of applications or propose a clean-slate design. In this paper, we present Retcon, a live update toolchain for embedded systems that supports a familiar event-driven programming model and does not require application code changes. Retcon leverages static analysis at compile time to determine when it will be safe to update a device. To find safe update points in the presence of complex asynchronous behavior, we define a novel system state, asynchronous quiescence, in which an update can be applied. We evaluate Retcon on a set of embedded event-driven applications – a dual-chamber pacemaker model, a programmable logic controller runtime, an artificial pancreas system, and a sensing node – and demonstrate Retcon’s ability to make low-overhead updates in less than one millisecond.