Anki does not have an explicit feature to combine decks, but you can accomplish this easily by moving all the cards in one deck to the other deck. Please follow these steps:
- Open the browser (click Browse at the top of Anki's main window) and select one of the decks you want to merge in the left pane.
- Select all the cards (Edit → Select All or Ctrl-A) and click the Change Deck option in the menu bar. Select the other deck and click Move Cards.
- If you want to merge more than two decks, repeat steps 1 and 2 as necessary until you have only one deck remaining.
- Close the browser to return to the deck list and delete the now-empty deck(s) by clicking the settings button to the right of each. (Anki will warn you if you try to delete a deck that still has cards in it.)
See also How do I move cards between decks?.