Yes, you can! You can change your username in your edit account section. Note that you are limited to only 2 changes, so choose wisely! Changes of case do not count as a change, or in other words, changing from 'paulo' to 'Paulo' will not count as a change.

Once you change your username, all the links where your previous username was being shown will be updated to the new one - this includes the 'Sent postcards' of anyone who would have to send you postcards.