This article describes how to set a Windows 10 application as the default application to open a particular file type.
This way when you double click on a file it will open with the application of your choice automatically.
For example we will change a file with .ini extension to open with WordPad instead of NotePad.
On the Windows taskbar:
- Click on the Windows logo
- Begin Typing: default
- When the application appears click on Open
When Default Apps window opens click on: Set defaults by app
Look for the application that is currently opening the extension you want to modiify.
In this example Notepad.
Click on the application name  and then Manage 
Application settings window opens.
Look for the extension of the file type you wish to modify.
In this case we will modify .ini type files.
A file of this type would have the extension set to ini, like this: settings.ini
Look for the extension on the right hand side [ 1 ] and click on the current default application that opens that extension. [ 2 ]
Once you click on the application the 'Set default by app' you can pick the new default app you want to use.
In the following example we are changing Notepad to WordPad.
Once you choose the new application you can close the open windows.
The the files with that extension will now open with the new application automatically.
[ On this example a file like settings.ini will automatically open with WordPad instead of NotePad when double clicking on it. Notice the icon ]