How to Edit Metafield Definitions
Once you create a metafield you can't edit the definition. But, there is a work around
When you remove the metafield-with-definition by clicking the delete button, you can choose between just deleting the definition or deleting both the definition and the metafield values.
When you choose the first option, you can select the metafield in the 'Metafields without a definition' tab. There you can click on 'Add definition' and set up the validation options in the way you want. This works fine for adding preset choices.