You need to have CC to save back.
Either someone with CC can do this or you can download the free trial for 7 days if your system supports it. Can you ask the creator to send you the idml?
you can download a trial version of indesign cc 2017 and save your file in idml.
or, somebody who already has indesign cc could save your file in idml and send it to you
Thanks Premio, yes I've already ran out of the trial so tried that avenue! May just have to upgrade
Thanks Jane, yes unfortunately don't have the facility for this currently so need a way of somehow opening through CS5 but doesn't seem to be one.