Follow the below steps to change the port
Step1: react-naive start --port=8082
Step2: Open the RTCDefines.h file and change the port RCT_METRO_PORT 8081 to 8082
Step3: Make sure two places you have to update in RTCDefines.h
If you get the same error when running from xcode, open project.pbxproj
replace 8081 to 8082