Open your Google Slides presentation and select the text you want to change.

Click on the "Font" dropdown in the toolbar at the top.

Choose a font from the list or click "More fonts" to explore additional options.

Select the desired font, and the text will update instantly.

To apply the font to all slides, click "Slide" > "Edit theme", then change the font in the master slide.

Close the theme editor, and the new font will apply to all slides.