@rrrraquellll I'm not sure if you can do it on your phone or not, but if you do it on the computer, go into "Create" and act as if you're going to write a new chapter. In the upper right-hand corner, there should be three dots or something like that. Click on that, and it should show options like delete, dedicate, cast, etc. Select cast, and you should be able to pick!
I hope this helped! :)