You can view all your previous (paid) Challenges by logging in to your account, then entering the URL (web address) for any 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: