You can change the minSdkVersionminSdkVersion in Project_Name/android/app/build.gradlethe file Project_Name/android/app/build.gradle , defaultconfigdefaultconfig :
defaultConfig {
// TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
applicationId "com.example.projectname"
minSdkVersion 16 // <--- There
targetSdkVersion 27
versionCode 1
versionName "1.0"
testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}