In your mobile app, navigate to your Wallet - you will see the different tokens you have been earning, and what your current token balances are.
Choose a token - tap to enter the details page for a token, and you'll see the redeem option for the token listed. Here we see the TRIBE token wallet which currently has three options:
To redeem tokens against a reward, 1. add the tokens required (or a multiple of if you want more than one reward) and then 2. click the "Redeem" button
It can take a little time to process as the app is performing a stellar transaction - but once done you should see a success message like this:
This is all you need to do - whoever is responsible for completing the redeem option you used should be in touch.