Upselling is a sales technique where you offer your customers the chance to purchase or upgrade to better features, better specifications or to get the more expensive version of what they’re buying so you can maximize the value of their purchase. 

To do so, Select such Alternative Products in the Sales tab of the product detail form. 3 alternatives are fine! Don’t publish too many otherwise your customers will be confused.

Turn on Alternative Products from the Customize menu of the product web page.