There is a challenge on the Candy Crush Game at the moment in which two players are paired with each other and have to collect a number of different items to win rewards. This is now called the DYNAMIC DUO.
If you have this on your game you can't choose your partner, but you can switch it if you have a buddy who does not appear to play.
To find the buddy challenge you need to click on the booster wheel icon on your gameboard and you will see if you have it there. If you don't have it there's nothing you can do.
If you have the buddy challenge you will see a picture similar to the one below.
The pictures show your own profile and your buddy, it may be a friend or it could be someone who doesn't play any more so it is purely random who you have to play with. If you are lucky you will have a buddy who plays and helps with the challenges, if you are unlucky your buddy doesn't play and you have to collect their quota as well as your own.
The number top right of the picture is how many challenges you have succeeded with. If you fail a challenge you will go back to zero and have to start again. The timer at the bottom tells you how many hours you have to wait until the next challenge or how many hours you have left to complete an ongoing challenge.
If you are not happy with your buddy you can change it, but you will go back to zero and have to start again with the first challenge, but it is worth it if you get a good buddy.
To change your buddy click on the "open" button shown in the image above and you will get see the image below.
Click on the button indicated top left and you can change your buddy. Some people have reported this not working very well and they never get a new buddy, but hopefully this will be fixed.
The rewards are not great to begin with but they get a bit better as you achieve more passes. Usually you will get 30 minutes of jelly fish or colourbombs, but as you progress you will get longer times and better boosters to play with.