how to Change your app's target API level in andriod studio

 Open build.gradle under android/app/

find the android { } block

Change the following version to the below:

compileSdkVersion 27
buildToolsVersion "27.0.3"
minSdkVersion 16
targetSdkVersion 27

In case your current android/app/build.gradle has lines which look like :

compileSdkVersion rootProject.ext.compileSdkVersion
buildToolsVersion rootProject.ext.buildToolsVersion

You will have to edit the android/build.gradle to include :

buildscript {
    ext {
        buildToolsVersion = "28.0.3"
        minSdkVersion = 21
        compileSdkVersion = 28
        targetSdkVersion = 28
        supportLibVersion = "28.0.0"
    }
    ...
}

• go to build.gradle file in your project
• look for defaultTargetSdkVersion & defaultCompileSdkVersion
• then change value to 26 

• Just open your build.gradle(Module: app) and change your compileSdkVersion and targetSdkVersion 26 or more than 26. 
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