You bow. In the old rite, people used to kneel to receive a blessing. If you can do that, go ahead. But if for whatever reason you can't, then at least bow your head.
As in 'Bow your head to receive God's blessings'. I don't remember hearing a 'Head's up' before the benediction is imparted. So, bow, at least.
Here's a good example:
Pope Benedict XVI, flanked at left by Cardinal Camillo Ruini and by his aide Rev. Georg Gaenswein, delivers his blessing at the end of a mass celebrated for the youths of Rome's universities, in St. Peter's Basilica, at the Vatican, Thursday, Dec. 14, 2006. (AP Photo/Gregorio Borgia)
9 hours ago