How to change the key of EasyPG?

[Water Lin]

I am using EasyPG to encrypt my document.

Here is the link: http://www.emacswiki.org/emacs/EasyPG

and 

http://www.easypg.org/.

After I used a key to encrypt the document, I want to change the key
which I think we can also call as password.

How to change the key or password?

Thanks

Water Lin

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