aws s3api restore-object --bucket [bucket] --restore-request 'Days=7' --key [filename]
However, the "File/Restore" option in Cyberduck does not work on this same COS instance - Cyberduck says "Failure to write attributes of [filename]. Standard retrieval is currently not supported. Please contact your web hosting service provider for assistance."
In case the issue was the "days" parameter (the s3api CLI command doesn't work if the "Days" parameter isn't set), I set the relevant hidden configuration option using
defaults write ~/Library/Preferences/ch.sudo.cyberduck.plist s3.glacier.restore.expiration.days 7
(I'm on a Mac) but this didn't help.