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Android:
Navigate to Settings > Connections/Network & Internet > WiFi (must be on to view known connections) > tap and hold the desired WiFi > tap “Forget” from the drop-down menu.
Note: if the WiFi you want to forget it currently connected to you will select the gear icon next to it and select “Forget” at the bottom of the screen.
iOS:
Navigate to Settings > WiFi > tap the again to confirm. 
symbol next to the desired WiFi > tap “Forget this Network” > tap “Forget”
