@Ali Salem 

How can I delete “Beta” water mark on the top right corner?

You can’t. But you could hide it with a white Shape, and even group the Doc and Shape, so the Shape moves with the Doc. As an added bonus with the Doc object, you won’t need to double-click to access it independent of any other objects it is grouped with.

Thanks a lot, but This “Beta” watermark will last forever? Any plans to remove it in the future? it’s weird XD

I expect that once the feature is out of beta, the beta watermark will be removed too.