Presenter normally picks up the Slide Title which is specified within your Slide content.
However, when no title is given for a slide, Presenter will default the title to Slide no. within your project.
If you want to explicitly specify the slide title, it can be done by modifying the "Navigation Name" field within the Slide Manager. (See below screenshot)
Note: Slide Manager option is available on the Presenter ribbon.
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Adobe Presenter Engineering Team