Q. Can I change an option's size or colour?

Yes - from 'Catalogue>Products' filter the product in which you would like to edit the option values for. Do not enter or edit this product, select 'SKU' from the available options.

Once in the SKU/Options screen, select the option size or colour value with the pencil icon attached.

Now, change the option to your desired value. You can change one options to follow the new value or all of the options within the product with the same value to follow the new value (the latter only used for colour).