Hi Samantha!
Thank you for bringing your question to the Disney Parks Moms Panel!
First, there is no penalty for not showing up for a FastPass+. It will just expire when the allotted time has passed.
However, you should be able to cancel for a single person. You can do it from the My Disney Experience app on your phone, or logged into your account on the
Walt Disney World website.
Go to FastPass+ on the My Disney Experience main menu, and then select the option to change or cancel a FastPass+ reservation. Next, click the check mark beside the person who needs to cancel. It will ask you to confirm this before it's official.
I found it to be a bit easier on my laptop, but it's possible on the app, as well.
I hope this helps! Please let me know if you have any other questions.