Yes — you can replace your training photos at any time, free of charge. Foxy will retrain your character with the new photos.
Steps
Open the Characters tab.
Tap the pencil icon on the character you want to update.
Tap Change photos.
You'll see your 3 photo slots: Face & chest area, Full body shot 1, and Full body shot 2. You can replace any or all of them — they don't all have to change.
Upload your replacement photos and save.
What happens after retraining
Your character is retrained on the new photo set
Future generations use the new training
Existing generations in your gallery stay as they are
You don't need a new character slot — retraining uses the slot you already have
If your generations weren't matching what you wanted, this is usually the fix. See My AI doesn't look like me — what can I do? for guidance on what to look for in better photos.