The credit card information won't be available to change until the payment has failed in the auto-charge. 

After the payment fails, you can change the credit card information by:
1. Navigate to Communities > Special Events > Friends Asking Friends > Kintera Thon 
2. Hover over Thon Name and select Bookkeeping
3. Under Add Batch Transactions, click Review Failed Auto-Charge Payments
4. Click Detail for a participant

A window opens that shows more information regarding the transaction. An Auto-Charge may fail for a number of reasons including the credit card may have expired or was closed, or the donor may have over extended their credit limit. Resolve these issues and then re-initiate the Auto-Charge.