Do not worry: if the size does not fit you and it feels like real discomfort, you have 14 days from the date of delivery to return the item that does not suit you, and choose to be refunded or receive a new pair in a different size - within the limits of available stocks.
You must then first revocate your order per mail, tchat, whatsapp or phone; see here all informations regarding the returns.
If you do not feel a too serious discomfort and you really want to keep your pair of Melvin & Hamilton shoes, you could contact your favorite shoemaker. He will surely have solutions for you to propose!