easy
log into your account on emirates website
then go to manage booking
enter the booking details of your friend
then follow the prompts for the upgrade.
Be careful though, if the flight you want to upgrade departs from UK, Lebanon or Pakistan you will have to pay the extra taxes, and you could only use a credit card with the traveller's name.

You could also, simply call your local skywards office and they will be happy to do the same.