How to reset the password on a Nikon WU-1a WiFi adapter?

From the manual (http://nikonsupport.eu/europe/Manuals/wu-1b/wu-1a_wu-1b_e5_01.pdf) you can reset the settings:

To restore default settings, keep the WPS button pressed for over 10 seconds and then press the button again when the LED flashes orange. The LED will stop flashing, showing that the adapter has been reset.

You will find the guide to connect to a Wifi compatible cellphone here: http://www.nikonusa.com/en/learn-and-explore/article/h4spl2xm/shooting-wirelessly-with-nikon-digital-cameras-and-wi-fi-adapters.html

It says basically that you just have to follow those steps:

1. Download the free “Wireless Mobile Adapter Utility” (WMU for short) app. from the App Store or Google Play™ Store. Once the app is installed, follow steps 2 - 6 every time you want to shoot with your camera and upload images using the WU-1a/WU-1b Wi-Fi adapter.

2. Insert the WU-1a or WU-1b into the accessory terminal port on the camera body and turn the camera on.

3. Press the button on the WU-1a/WU-1b, holding it down for about 5 seconds, until the LED on the adapter flashes.

4. On your smartphone open the Wi-Fi settings and select the Nikon camera’s network from the list of networks available. It will begin with Nikon, followed by a string of letters and numbers. For Android™ smartphones, once you initially make this network connection, your device will automatically connect when you enable the camera's Wi-Fi. For iPhone®, iPad® and iPod touch® users, you'll have to perform this step each time you wish to use the wireless feature of your Nikon camera with your smartphone.

5. Once the connection is established, the LED on the adapter will glow green.

6. Launch the WMU app. Upon selecting the Take Pictures Remotely option in the app, Live View is started automatically, allowing you to now see what the camera sees. Upon selecting View Pictures, you can view the photos stored on the camera’s memory card and choose those to download to your smartphone.

Step 3 seems to be the one you are missing.