Which slide should you change so that it reflects on all the slides of your presentation?

Hey there!

To edit your presentation to where changes to the fonts, images, colors, etc. will affect all of the slides in the presentation, you will need to edit the Slide Master. You can do this by either selecting a new theme for the presentation (all of the slides from one theme should change to the new theme) or edit the master slides from a preexisting theme to customize the slides. 

Hope this helped you out! :-)