SHELL

INSTALL_FAILED_UPDATE_INCOMPATIBLE: Package signatures do not match previously installed version; ignoring!

Just run ADB and use the following command:

adb shell pm uninstall -k --user 0 <package name>

INSTALL_FAILED_UPDATE_INCOMPATIBLE: Package com.*.version signatures do not match previously installed version; ignoring!

cd android &&  ./gradlew clean

npm start -- --reset-cache ( if you are using npm )

react-native run-android

if you are using yarn then after clean run 

yarn android

INSTALL_FAILED_UPDATE_INCOMPATIBLE: Package signatures do not match previously installed version; ignoring!

You need to uninstall previous version from it because you are using a different
signature than the original. If it is not working it might be because it is 
still installed for another user on the device. To completely uninstall, 
go inside emulator

Settings -> Apps -> Hagnostic News -> Options (the three dots on top right) 
-> Uninstall for all users

INSTALL_FAILED_UPDATE_INCOMPATIBLE: Package com.package signatures do not match previously installed version; ignoring!

Just in case if somehow your old application is not removed 100% or its data is not removed.

Try:

adb uninstall "com.yourapp.yourapp"