This forum helped me figure it out, but it took some fiddling. If anyone needs clearer instructions:
1. download your new deck and unzip it
2. in Anki, go to file > import and import your new deck
3. on the main Anki screen, there should be “Decks” “Add” and “Browse,” select “Browse”
4. a new window should pop up, on the left side there should be a list of your decks/tags and such, select the deck you just downloaded and do select all (command + a on macs) to highlight all of the cards within that deck
5. click on “Change Deck,” which is at the top of the window along with “add” “info” “mark” “suspend” etc., and choose the deck to move those cards you selected to, click on “Move Cards”
6. you’re done! Now just delete the now empty deck you imported because all of the cards from that deck were moved to the one you selected