You can easily edit or add variations to your items. Just follow the steps:

1. Click on the highlighted V button at the end of the line for each product or if you’re in the item already, click on Attributes -> Variations
2. 
3. Click on the Add button to add new columns or Edit to edit them
4. Fill in the empty fields with the right info
5. You can now press the Save button and Post it or Revise it

NOTE: To add variations to an Item, it needs to be set on an eBay category that allows that. You can distinguish them by the [V] sign they have at the end of the category