On an Up-sell popup, you can choose to redirect buyers to the cart page or checkout page. You can choose to redirect them to cart page or checkout page as your scenario.
In order to do this, please visit the app Settings >> Up-sell Settings >> "Call-to-Action button will motivate customers go to" and choose either "Cart" or "Checkout".
Tip: By ticking the option Use Boostflow cart popup, Boost Upsell will use its own cart which can make the shopping experience better and less time-consuming. On the Boost Upsell cart, your customers will see recommended product for easy up-selling. Note though that if you are using Last-step Upsell offer already, it is better to disable this since the Last-step Upsell offer can only be triggered by clicking the "Checkout" button on a specific cart page.
Click Save and you're done.