To make changes to the format of all your slides at once, using the Slide Master is probably the best idea. The Master is what controls how your slides will look: what fonts, colors, bullets, etc. that will be used on each slide.
With Slide Master, any of the formatting for your presentation can be changed at once and then applied to all slides immediately; for example, you can change the font of the title or text, the type of bullets used, add an image to the background, add an image to every slide, etc. and once you close the master, this format will be in affect on all slides – including the ones you later add. To enact these changes, you should: make sure you are on the Main Slide Master (the one at the very top in the left column), then highlight the Click to edit Master title style text, click on the Home Ribbon and choose a font, size, and color from the Font Group, then click on the Slide Master Ribbon (to the left of the Home Ribbon) to return to the Slide Master View. If you are finished, click the Close Master View icon at the right end of the Slide Master Ribbon.
To access the Master Slide, go to the View Ribbon, and then click on the Slide Master icon in the Master Views Group.
Note that when you enter the Master View, you land on the layout of the slide you were on (in the example below, the Title Slide layout). The problem with this is that any changes you make will only affect slides that use that layout; if you want to change the title font on all slides – regardless of the layout used – you need to click on the big slide that is at the very top of the list. Then your changes will affect all slides.
If you want to change one of the other bullets, repeat the above steps.
When you are all finished, click the Close Master View icon at the right end of the Slide Master Ribbon.