Hi Lucinda,
You should be able to exchange currency into Krona at the banks or withdraw from an ATM. Major credit cards are accepted in hotels, larger shops and restaurants but please notify your card issuer before you depart to avoid any problems. Euros are NOT accepted in Iceland.
I travelled to Iceland earlier this year, and from my own experience it was difficult to exchange currency over the counter. I ordered mine from the Post Office, a friend ordered their through their bank. Both took a couple of days to turn around the request. Others in the group changed their money at the airport before we flew.
I hope this information helps.
Kindest regards,
Julia