You can view your previous (paid) Challenges by logging in to your account on the website, then entering the URL (web address) for a previous Challenge you've done.
When you log in, you'll be taken to the current Challenge automatically.
The URL will look like this if you haven't paid yet:
or possibly like this if you've advanced to the payment screen:
If you've already paid, it will look like this (and you will be taken to your dashboard):
From there, you can simply click on any link below to go to the Challenge of your choice: