From your Nearpod Library, go to the lesson whose slides you'd like to copy and go to edit it. Select the slides, copy them with your keyboard or the buttons above the slides.
Go back to the Library, and select the lesson where you'd like to place them, select Edit. Paste the slides there either with your keyboard or with the "Paste (Ctrl+V) button. You can drag and drop them to rearrange their order.
*Please note that with the current implementation, you can only copy slides from lessons that you have created yourself.