To Forget Wi-fi Network in Windows 10.
1. Close/disconnect Wi-fi connection.
2. Click the Windows logo / Start Menu , or press Windows logo on the keyboard
3. Choose Settings.
4. Choose Network & internet.
5. Choose Change adapter options
6. Choose Wi-fi icon
7. Choose Manage Wi-fi settings
8. Scroll down to find the SSID/wi-fi name that you want to forget
9. Click on the SSID name , then click Forget
For Windows 10 with new updated. for step 7,8,9 are different :
7. Choose Manage known networks
8. Select the Wi-fi/ SSID that you want to forget from the list.