Here, select a presentation that's already saved in Google Slides or choose a presentation to upload from your computer. Click Select.
Choose which slides in the presentation you’d like to import. Use the Select Slides: All option to quickly select all slides, or manually select slides that you’d like to import.
Leave the box next to "Keep original theme" checked if you'd like to import your slides unmodified. Uncheck the box if you'd like the slides to fit into the look of your new presentation.
Click Import to finish.