Is there a specific field you’re having issues with? To delete on of the Available Options, you can click into the custom field by visiting Account Settings > Custom Fields > and clicking into the Custom field. When you click into the custom field you should see a trashcan next to the item.
If you are looking to change the Field Specification itself. E.g, switching the field from a ‘Single-Select’ to ‘Multi-Select’ field, you will need to delete the entire custom field and re-create it. You cannot change the field type on an existing Custom Field. To delete an entire Custom Field, select the checkbox next to the Custom Field and click on the Delete button.
You can see more details in the Custom Fields unit: https://hitchhikers.yext.com/tracks/knowledge-graph/kg122-fields-intro/03-create-custom-field/.
Let me know if that helps, or if there is a specific field you’d like to dig into!