Hit "My Account"
and "My Banknotes"
At the bottom of your list you will see this:

You should hit the BOTTOM 2nd link (below) so you can view just the countries you've entered:
Once you find the banknotes you wish to swap, you'll want to edit your listing:

The "edit icon" is the little "diagonal doo-hickey" next to the "garbage pail" icon. Hit it (not the garbage or delete icon).
It will then show the box "irruig" has posted, and you can change to "yes" for exchange.