Use variants if you have a single product that comes in multiple sizes, colours or other variants.
On any product page:
Click the Variants tab
Click the Add product variants button
Enter the name of the variant in the Options name field
Add as many variants as required in the Choices field
Select the Reorder dropdown values button
Click and drag the value up or down the list
Select the Pencil icon next to any value
Toggle Visibility to hide or show a value
Select any option to edit
Adding additional options will remove all existing variants.
All price, weight and stock information is deleted along with the option