Creating a custom field a client can edit in theme's header file

Hey guys,
I’ve got a couple of phone numbers in the header of my theme (I’m using Blank). I need to make sure that these can be manipulated by the client from the back end. Sort of like uploading a custom logo, but for phone numbers instead.

I’ve been searching all around the place, with not much luck yet. Could anyone suggest some kind of tutorial or even just an idea of how I can start going about doing this?

Seems a little out of my league, but I’m pretty much learning strictly by trial and error, so any and all help would be greatly appreciated. Thanks!!