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JavaScript

var jsonObj = [{"devices":{"dID":"TLSM01"},"uuid":"e863c776-f939-4761-bbce-bf0501b42ef7"},
 {"devices":{"dID":"TLSM01"},"uuid":"5a0cd70d-891d-48d8-b205-e92e828ac445"}];
 
jsonObj.map(item => item.devices["EntityID"] = "12458412548");

console.log(jsonObj);