Probably a very basic question re menu items

I’m a complete newbie when it comes to WordPress, so apologies if this has been answered a million times. I’ve done a search in the forums for answers but I’m not sure if I’m even using the right terms in the first place.

We’ve installed WordPress on a site we’re setting up and are using the non-profit Organic Themes theme. I don’t think it’s an issue with the OT theme – more just my ignorance. I’m wanting to sort out the tabs you’d have at the top of the site below where the logo would go (e.g. Home, Contact Us, About Us, etc). How would I go about doing this?

I’m sure it’ll be really obvious but I feel like I’ve clicked on every possible option and I’m still not working it out. I’ve wondered if I’d need to amend the CSS code in order to do this, but I’m hoping it’s nothing advanced like that.

To be honest, I’m not sure if I should even be referring to them as menu items, hence why I may not have found any answers.