How can I add a variant dropdown to my product page?

[Karineyf]

Hello everyone, I'm new to Shopify, I need to add a dropdown menu as variant selector on my product page, 

for example when the user is trying to order a ring, he can select the ring size from the dropdown menu.

i checked other similar discussions about this, the answer was : 

" select "Product pages" from the drop down menu at the top of the page, under "Product Options Form" change the "Picker type" to "Dropdown""

that's not available on my product page, is this related to themes? how can I fix that?