In the back office we need to navigate to the item we want to add an upsell prompt to. To do this go to Menu under Menu management.
The item we're going to add a prompt to in this situation is Peanuts, you can see we searched the category to quickly find the item.
Click the edit button on the item and scroll to the bottom, press the edit button again as shown above.
Scroll down slightly until you see the options above. Here we've added a prompt to say go large £4.50, make sure to change the option above the prompt text (Prompt for upselling/checks) to 1 from 0. Remember to Save & Return once you are finished.
This is how it will appear on the POS front.