How do you merge decks in Anki?
Open AnkiDesktop.
Open the "Browser".
Click on the deck you wish to remove on the left-hand pane.
Hit ctrl+a (command+a on mac) to select all cards.
Click "Change Deck" at the top.
Select the deck you want to keep in the pop-up dialog.
Click "Move Cards".
Repeat steps 3 - 7 for all decks you wish to merge.
Go back to the "Decks" screen in AnkiDesktop.
Find the empty decks (the decks you wish to remove will have a card count of null).
Delete those decks.