Yes this threw me off when I first opened it. When you select the letter field you see a flashing cursor but you can't erase the character and type a new one. Instead you have to type a new character which overwrites the old "editing" character. This is confusing because it's not the behavior that the user expects when they see a flashing cursor in a text input.

I would suggest allowing people to erase the letter and type a new one in the editing field.

Once I figured out this little UI hiccup I found it absolutely delightful to play with this. What a fascinating experiment in making a font immediately editable, like a mini font-REPL. I've often been interested in (but never dabbled in) creating or editing fonts. This made that itch immediately scratchable in a raw, primitive way that unlocked something interesting in my brain.

This is a great example of what art can do.

Where you see the 'Q', it's a text-field, you can just enter a different letter and it'll switch.

The letter below the "Editing" label is an input field. You can change the current letter there.

Type the letter you want in the left box.