If we want to install an apk already downloaded or we compiled, use below command to install the same using adb as,
$ adb install com.android.helloworld.apk
Here, com.android.helloworld.apk is an application APK name, so you can use it yours as per name of file.
$ adb install APK_FILE_NAME.apk
If we have already installed this apk previously and now want to install another version on top of this, then from adb use below command which will remove already installed application and install the new as,
$ adb install -r com.android.helloworld.apk