Ted's Spelling correction

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 5th level or lower, you can choose a number of letters to replace or remove within the spell up to the spell level of this spell, the effect of the spell changes according to the letters replaced or changed, if the spell contains no letters or the letters do not spell a word, its spell fails and has no effect. The DM determines the new spell's effects.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the number of spells you can choose to change increases by one for each spell level beyond 5th.