As I mentioned in another thread, my registry associates .txt with the HKCR\txtfile key; yours may be different: you can look in HKCR\.txt to determine the right association for your machine. Assuming my setup:
You’ll notice the three keys “open”, “print”, and “printto”: mine does not have an “edit” key. So if they are really using the hardcoded “edit” key, rather than the default action (listed here as HKCR\txtfile\shell:(Default)=open), then it’s a poor choice on their part, because there is no guarantee that the “edit” key exists for a given association on a given machine.
!!WARNING!! What I am about to explain involves editing the registry. This has associated risk, and should only be followed if you understand everything said, and are willing to take all responsibility for any change in behavior that occurs in your system. I make no guarantee or waranty regarding the effectiveness or safety of the following. (If your computer stops working, or doesn’t work the way you want it to, it’s not my fault.) Use at your own risk.
You may be able to work around their choice. Right click on shell, select New Key = edit. Right click on edit, select New Key = command. Left click on open > command, double click on (Default), and copy the path to NPP (if your default isn’t “open”, but some other key, then use the command from that key, instead); left click on edit > command, double click on (Default), and paste the path to NPP. Exit the registry editor. (Some people say that they need to reboot, or at least log out and back in, or perform other similar actions to make sure the registry has fully updated… but my guess is just closing the registry editor and closing/reopening the third-party program will be sufficient.) In general, this should create the new “edit” action, which will execute the same command that the “open” action executes.