How to set android WiFi hotspot password on Android 4.1.2
To access the hotspot settings you have to keep (tap-hold) your finger on the same field Portable Wi-Fi hotspot for a few seconds, then release the finger. 
Unless there's a menu icon (three dots) on the top right corner, in which case you can access the settings by tapping on the menu icon and choosing Configure. 

Generally speaking, in most apps you can tap-hold on a field to pop up a list of options. This is not very intuitive at a first glance, but after you've used Android for a while it becomes natural.