You right click a file of the type you wish this new editor to open. So if you want this editor to open text files right click on any text file, then when you choose a program from the Browse button, it adds it to the list of programs in the Open with window, if its not selected then, you need to choose it, then check the Always open files of this type with this program and click o.k. at that point the icon for the file should change to reflect the new application.
You need to have Admin privileges to be able to do it.
If that won't work, you can do it from the Tools/Folder Options menu. Click on File Types, and find the extension of the file type you wish to change. Select it, and click either on Change or Advanced.
If you click on change it will open the Open With Dialog.
If you click on Advanced it will give you more options to modify.
Click on open and then on Edit to change the application that will open the file.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.