Use this code in your buildscript (build.gradle root):
subprojects {
project.configurations.all {
resolutionStrategy.eachDependency { details ->
if (details.requested.group == 'com.android.support'
&& !details.requested.name.contains('multidex') ) {
details.useVersion "version which should be used - in your case 26.0.0-beta2"
}
}
}
}