There isn’t really an “end” to UNO, per se. The game is designed to be played until one player has reached a pre-determined score (usually 500 points). Once a player has reached this score, the game is over and that player is declared the winner.
However, if you wanted to end the game on a specific card, you could agree with your fellow players that the game will end when someone plays a power card. A power card is any card that has a special effect, such as making the next player miss a turn or draw two cards.
If you’re playing with younger children, you may want to set a lower score limit so that the game doesn’t go on for too long. Alternatively, you could agree to end the game after a certain number of rounds (i.e. each player has had a turn). Whichever way you choose to end the game, make sure everyone is agreed on the rules before you start playing!