I think there's a problem in some of the terminology being used ;-)

A PSD a design created in photoshop and it is only a design. That design needs to be then 'coded' into html/css by a coder so that you have a functioning website.
Is the person you refer to a coder - will they take the psd and code it for you - not just update it.

You really need to know this, at the moment, 99designs does not have a coding category, so this contest will be for the design only and the deliverables would be a psd file, which you hand over to a coder to work with.

hope this helps :-)
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SitePoint Community Mentor