Windows 7: i need some help, please

Can you explain me, please, if it's possible, how to change the extensions ?
I have a lot of videos / clips , and they are in different extensions, like .avi / .mpeg / .vmw / .flv and so on
I wanna open them all with my KMPlayer. I tried this way :
right click one file, properties / change / and i selected KMPlayer. But that's only for this kind of file.
Thanks.

These may help.
Open With - Change Default Program
If you want to change the Associations, this will suit your needs
Default File Type Associations - Restore

You cannot just arbitrarily change extensions. You must be careful or the program will not work properly.