Simple utility to edit wifi connection credentials in a raspbian system

I have a Raspberry Pi 3 B that has a built in WiFi adapter. Mostly I use the Pi as a headless system to read some sensors. The Pi doesn't have a display, I haven't connected one on purpose as I want the unit to sit at the corner of my room and run the sensors only. The Pi communicates with the internet using my home router. However, every time I change my router's password or I use my wifi dongle incase the router is down, I have to SSH into the Pi using an Ethernet cable and manually edit the [login to view URL] file and add the new ssid and password for the connection. This file contains the fields for SSID and PSK. I want an utility(a shell script running from [login to view URL]) to be made such that, whenever I plugin a pen-drive to the pi, the pi looks for a file named [login to view URL] in the pen-drive. In case the file is not found, no action is taken. If the file is found, the contents of the file are parsed. The file needs to contain the new SSID and PSK. These two are parsed, and put into the [login to view URL] file and the pi reboots and connects automatically to the WiFi. In this way, I can connect the pi to the internet using any WiFi connection. All I would need to do is to add the [login to view URL] file containing the new SSID and PSK to a pen-drive and plugin the pen-drive to the pi. The pi then parses and configures the WiFi setting by itself.