Drat, you've named your character Johnathan and just now realized that Johnny would've been better. We get it.
Because of the browser-based design, we can't give you a find and replace option just yet. BUT, did you know your browser has a find function built in? On Windows just use CTRL+F and on Mac CMD+F to find all instances...you will have to manually type in the replacement, but a little copy-pasting will make that easier!
Celtx handles auto-completion, formatting and spacing of Character names, so while you still have to Cmd/Ctrl + F to find each instance, Celtx will take care of some of the work for you!