Fenix 5 Plus - changing maps from Europe to USA

I live in the UK and I'm going on holiday to the US (New England) in September for a week. I've already found DC Rainmaker's instructions on how to download the maps from garmin.openstreetmap.nl and have the e-mail with the link to download the maps. I'll be using a MacBook for the download.

Couple of questions:
1. Do I need to download BaseCamp on my Mac first?
2. Do I have to delete the existing European maps, or can I load the US map tiles (there's only 6) as well and then choose between which maps I want to use, depending on where I am?

Thanks in advance