Adding, modifying, or removing a letter from the letter library is simple; just remember to never type client or account details into these letters. That all happens in the Dispute Wizard. The library is just for storing the templates.
All the letter templates can be modified, but there is one letter you cannot replace. It's the Default Round 1 Letter (to bureaus). This special letter has backend coding, so it's the only letter that will send for Wizard 3 Round 1 default. You can certainly modify it, just be careful to save a copy first and do not remove the text in brackets (as those make the data merge work).
Here's a video to show you how to add or edit letters in the library: