If you're using the Android app, it should prompt you describe the process. First hold down A + B and press reset at the same time. You may need to hold them for a few seconds.
After the word 'PAIRING MODE' scrolls across the LED display you should see a pattern displayed.
Copy the pattern into the app
After tapping PAIR you should receive a success message and a tick/check mark appear on the LED display.
You have now successfully paired the micro:bit. Press reset to complete the process.