Some time ago I changed the way my computer reads .dat files by forcing it to always open with KM player. Now, if I save a .dat file in my laptop, its properties says that its a KMP-MPEG file. I want the type of the file to be .dat only. I have a license file that has to be in .dat format. But the software, for which I have the license file, is not able to read it. How do I change it back?
It shouldn't matter what the OS says the type of file is.
As long as the file is the correct format for your software it should be able to read it.
If you are talking about Windows:
About "nature of file"
You have to know that the system does not read the contents of a file to classify it in registred filetypes, but only file extensions.
So, if you have a video file originally saved as .AVI, if you rename its extension to .TXT, Windows will think is is a text file. Some softwares are able to recognize filetypes by its contents, whether or not the file extension is correct (e.g. Irfanview, Media Player Classic...).
I.e, the "nature of your file" is preserved whether or not the file extension is not correct, EXCEPT when you use a repair software (or a software that try to repair a file), that could try to convert the contents of a file to the match extension format, and so it will corrupt a file that does not match current extension (for example, if you have a .JPG and rename it to .DOC, a registered program to open .DOC extension tries to recognize it as text; if you save the result, it can generate a corrupt file, no matter you change extension back to .JPG).
How to change registred filetype
Just use "open with", choose your program and mark to use always that. For .DAT files, there are a lot of programs that use them.
How about your license file
What means to read your license file?
The shell settings for a file extension do not affect whether or not a file with that extension can be opened. You have other problems, such as a corrupt file.