Example of a Product shown in the VIEW PRODUCTS area with a variant shortcut button to quickly get you to the variants settings area.
The Options/Sizes screen will show the variants listed vertically. Update the price for the required variant via the PRICE column as indicated.
You can also update the main product price (e.g. the price if no variations are chosen, default price for variations) from this screen by clicking the EDIT PRODUCT link in the small tabs above.
This will take you to the main product detail page where you can update the price.