As an admin, search for the user, and you should be able to do it when viewing their profile (I hope we don’t require the current password for this).
Otherwise, via indico shell:
identity = Identity.query.filter_by(identifier='usernamewiththespace').first()
identity.identifier = identity.identifier.strip()
db.session.commit()
Also, it would be good to know how the user managed to get a space there - AFAIK we strip surrounding whitespace everywhere, so if there’s a way to create an indico account with surrounding whitespace in the username that’d be a bug.