If you have forgotten the password of an already joined wireless network, you can easily retrieve saved Wi-Fi password in Windows. To view passwords for saved Wifi networks in Windows 10, do as follows:
1. Make sure you are connected to the wireless network, you want to view the password
2. Right click on the Wireless Network button in the system tray next to the time and date in the bottom right of the screen
3. Click on the Open Network and Sharing Center option
4. Now, as shown in the below, click on the wireless network's name you are already connected to
5. The Wi-Fi Status windows will open and you should click on the Wireless Properties button
6. The properties window for your wireless network will open, and here you should go to the Security tab
7. Some information about the security of your Wifi network is shown, and to view saved Wi-Fi password for this network, simply check the Show Characters option, and see the Network Security Key item.
Note that, you must have the administrative access to get the password for save Wifi networks in Windows 10.