There are a few places in which you can add an upsell: on the cart page, the product page and post purchase. I’d recommend taking a look at these apps as a start to determine which would work best for your business needs:
Please reach out to the app developer directly if you have any questions about how an app works, or need help setting it up. If there is one in particular that you are interested in, feel free to take advantage of the free trial time and install it on your store to test it out to see if it functions as you’d like it to. For any apps that are paid, just remember to uninstall them before the trial ends if you do not want to use it so that you don’t get charged for it.
Hope this helps! Let me know if you have any other questions.