If you have available Extra Spins, they will show up next to all eligible transactions in your ‘Recent Transaction’ list. Transactions made in the last 24 hours are eligible to be respun.
Click the ‘spin’ button next to the purchase you’d like to use the extra spin on. You’ll be asked to confirm you want to use this before being taken to the purchase spinwheel. From there, you’ll go to the current purchase wheel to spin and (hopefully) win big!