Customer Access?

Okay I’ve tried, but before I really mess things up I thought I’d ask. I have 4 sites on my account. I want my customer to be able to edit one file on one of the sites ( a simple text document ). How do I get him access to that file only, so that he can edit it, place it back on the site and not touch anything else? Is that an account thing or User thing and if so, what are the steps? Thanks in advance

How much “other stuff” is there on the site?

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On his site there is an entire website (specifically several html docs and a few flash movies, images, etc.). Typical site.