need information about how to implement OAT update in android

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i have an android application in this i am using NRF lib to connects to BLE device and getting value suing service and characteristic now i am working on OTA update function so need more information how we can implement that in android application,

i am using this DOC. as reference to write a code but OTA functionality not working, can you please help me to implement this functionality in my code ?
thanks in advance.