You’ll find a menu bar that runs along the top of each slide in the Slide section of the app that has the Bold, Italicise and Underline options.
To use one of these buttons, highlight your word(s) and then click the button.
Alternatively, you can highlight the words and use these keyboard shortcuts:
Bold = Control + B (Command + B for Mac)
Italics = Control + I (Command + I for Mac)
Underline = Control + U (Command + U for Mac)