It is not currently possible to adjust the step titles in the Travel Book Editor. 

If you'd like to change the title of a step, you must first do so in the trip itself.

To do so, go to the trip in your account -> find the step -> tap 'Edit' -> adjust the title -> publish step. Have a look at this Video Tutorial for a quick demonstration.

Then, next time you generate your Travel Book preview the step title will appear as you would like!