Hi, chiara. If you go in to any shop where they repair shoes (possibly Timpsons?), they sometimes sell a shoe stretching spray. Once you've sprayed the insides, wear them straight away for a while and they really will stop pinching. :)
Shoe stretching spray can be bought from Clarks for a few pounds, then wear the shoes with really thick socks.
Alternatively find someone with bigger feet than you and ask them nicely to wear in the shoes for you !!!
Shoe repair shops also have stretching machines, they put the shoes onto the stretch mould, where they are gradually stretched over a period of several days.