Hiya Newbie,
I guess the real issue here for us to answer that is our morals. I don't really think it's appropriate for us to start teaching you how to do things if someone else is working on your website. If you have issues with them working on your website then you really should take that up with them. I'll assume you are paying them to do your site, if that's the case then you have every right to request information on how to edit things and you have that right to ask them directly. If they are doing it as a favor, well you still have that complete right, but hopefully in either case you'll grant some patience for them to get it finished before getting too worried about editing it.
It's a tight spot for us to stick our noses in between a customer and their web developer. I hope you understand that, it's not a matter of we "can't" or "won't" it's that at the moment we shouldn't.
Having said that, I'd like to tell you one of the main reasons why. Since you are entirely green, I can totally understand your website builders issues with explaining how to do things. It's not a matter of point and click here and the work is done. There's a lot more involved with it than that, and playing with menus and pages (which are easily messed up and accidentally deleted) are some real concerns with people setting up customers to edit their own sites after they have been built.
I am very sure the builder will sit with you and explain how it works and what you should and shouldn't do, but do have patience, it's not rocket science, but it's not Kindergarten either, it takes a fair bit of explaining to show someone with no knowledge at all how to do this, and especially to show them with them understanding what and why they are doing it.
Without us knowing what type of contract you have worked out, it wouldn't be fair for us to butt in and override any agreements you've made with the site builder. Talk to them about your concerns and work out what and how you will be editing it as they may have plans for how they intend to set that up.
Sorry to rain on your parade, and I'm sure in the end it will all work out, they are probably just worried about making sure you know what you are doing before you get in there doing it, and I'm sure most here would understand that.