convert itune movie I purchased to other format so that I can use other player to play

In reality you didn't purchase the movie, you purchased just the rights to view the movie in iTunes.  We are forbidden from telling you how (Terms of Service) but I would think that perhaps others want to do the same and perhaps google search may help you 

I have been able to watch iTunes content in full screen on my computer. You can also attach a TV or projector to your computer via HDMI, or stream to an AppleTV.

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If the movie is "Movies Anywhere" enabled, you can link accounts and watch it via a different video service.

Only available for US residents
Eligibility. Only legal residents of the United States, all U.S. territories, and the U.S. associated states of the Federated States of Micronesia and Palau (collectively, the "Territory") are eligible to register for a Movies Anywhere account.. 

Gerwin Jansen ,

I see you already picked a solution, just sharing that on Windows to play full screen in iTunes, you can use the key combination Control-Shift-F - the same for switching back to windowed. 

what I want is the view full screen in other player. What if itune is not working after I migrate my PC to another hardware..... something strange can happen.

Itunes is an application, you can always install.

As was pointed out earlier, conversion of DRM protected content is contrary to EE AUP.

try if you go to icloud.com and login with your appleid can you see the itunes items ?

Accesing the web portal, might be an option though you have to know whether and what the web URL is.

I would not worry about migration to other hardware (see above comment by Arnold). You can try the Apple TV app (or device) it may be installed on a smart TV (if you have one).

tks all

tks all.