A minimized icon will appear on your store when your customer accidentally closes an upsell offer. Customer can simply click on the minimized icon to go back and view the upsell offer.  

It will display the first image in the upsell offer as a minimized icon on the right bottom corner of the product page. 

By default, the minimize icon is enabled. To disable it on your page, go to Settings >> Other Settings >> Minimized Icon >> Untick Show minimized icon on mobile >> Save.

That's all and you are good to go.