How to change file extension super easy way

What is an extension?
According to the dictionary.com extension means

The state of being extended, lengthened, or stretched out. 

According to tech term

a file extension (or simply "extension") is the suffix at the end of a filename that indicates what type of file it is. For example, in the filename "myreport.txt," the .TXT is the file extension.

Now if you want to change the file extension you have to do the following steps:
(For Windows users )

1.  Go to your folder option
2. Tab to View
3. Uncheck "Hide extensions for known file types

Then select your file and right click on your mouse and choose rename from popup list.
Change characters after a *.*
For example Your file if myreport.txt
if you want to change it you can do it easily

I have a video instruction also you can see here if needed :