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If no online check in is available then the only way to check in is at the airport.
According to Iceland Air website it is possible to choose seats during the booking process, however there is no info on seat change. It might be possible to change the seat on the website\by calling an agent however the seat you choose is not guaranteed until check in according to several threads including this one.
The process for choosing a seat online is detailed in this thread by a customer.
I have found on the site where to request seats (I DO understand that it is a request only and have seen on previous forum answers that the seats can change).
I went onto Icelandair's home page and clicked on 'My Trips'.
Enter your flight reference and surname and on the next page you will see 'seat preferences' in blue. Click on that and follow the instructions.