ADB and Fastboot are included in Android SDK Platform Tools. Download the package for your computer from the Android Developers release page. Android Studio users can also install it through SDK Manager.

Open the extracted folder

Extract the ZIP, open a terminal in the platform-tools folder and check that the executables run. In Windows PowerShell:

.\adb.exe version
.\fastboot.exe --version

On macOS or Linux, from the same folder:

./adb version
./fastboot --version

A version result checks the tool on your computer. It does not mean that a phone is connected or ready for an update.

Check an ADB connection

For a phone using normal Android USB debugging, enable USB debugging, connect a data-capable cable and accept the authorization prompt on the phone. Then run:

.\adb.exe devices -l

Use ./adb devices -l on macOS or Linux. A listed device is an ADB connection; offline means that it is not responding. If no device is listed, check the cable, USB mode, debugging settings and computer driver. Google documents these states in its ADB device-query reference.

Recovery sideload and Fastboot use their own device modes. Do not unlock or reboot a phone just to make a command return a result. Use the mode required by the manufacturer’s procedure.