First, go to Products > Select the product that you would like to edit and scroll down until you see this area:
From here, enter a label that will appear with the options, I have entered Size as an example,
After that, click in the middle box and begin typing your first option. Once you have typed the option, press "," to confirm this and repeat the process.
Finally, scroll down beneath the options you have just created to edit each individual variants price, stock and weight if necessary. As an example I have edited some of my size variants to show different prices.
After you have finished editing, click Save in the top right of the page.