htaccess redirect to maintenance page but not if they want a page inside of specific folders

Dear Experts,

I am needing to do a few updates on the public portion of my site and need to redirect traffic using htaccess to a maintenance page while I update the pages,  I don't really know where to begin. I am hoping it wont be to complicated but what I need to do is this:

redirect all traffic to a maintenance page called "update.php" unless they need access to a page inside of a folder called "cora" or "member". Also I would like to allow them access to the login and the logout page which is "login.php" and "logout.php"

Also I would still like to allow myself access to the site while I test to make sure the update is working correctly. So I would need to allow my ip address have access to everything.

Many thanks as always