There are two ways how to access Challenges.
Via the Home screen
(The tiles on your home screen are customizable based on your preference. You can click 'More', tap the Personalise shortcuts' button on any of the tiles and click on the + sign icon on the Timer icon to add it to the list.)
1. Go to the tile icons on top of the Home screen.
2. Click on More.
3. Go to Challenges.
Via the Library tab
1. Click on the Library icon at the bottom of the screen (beside the Home icon).
2. Click on Challenges.