Once logged in to your Shopify Admin page, select "Apps" located on the left side of the page.
In the list of installed apps, select Smart Upsells - Upsell by Surge Apps.
Next is to select the "Settings" button located at the top right side of the page.
Scroll down to the last setting called "Minimized Button Settings". This is where you can customize the color, text and position as well as enable/disable the button.
If enabled, this will only appear when a customer minimizes the upsell offer box.