Press and hold the top button for 5-7 seconds. 

​If the LED light blinks continuously:

That means then pen is receiving power and is trying to pair with a device. Follow the steps to pair Surface Pen with your Surface.

1. Select the Start ​ button, then select Settings > Devices > Bluetooth & other devices. 
2. Make sure your Bluetooth is On, then select Add Bluetooth ot other device > Bluetooth. Choose Surface Pen in the list of devices. Follow any other instructions if they appear, then select Done. 

If that doesn't work, Restart your Surface and try pairing again. 

If the LED light blinks ofr 2-3 seconds: 

That means your pen is already paired to another nearby Surface. 

To fix this, find the Surface and Remove the Pair. You will need to look for Surface Pen in the list of Bluetooth device. After removing the pairing, go back to the Surface you want to use Surface Pen with and pair it again.