I believe the instructions for how to set the date are posted at the UN website, but even if not, a set of simple instructions are in the excellent article written by our own Marcus Hanke right here:
I am fortunate enough to own a GMT/Perpetual and it has become one of my favorites. Setting the watch is quite straightforward. However, if the watch were not to be worn for a few months (or a year, god forbid!), what is the best way to advance the month,...
When the crown is pulled out to its first setting, about halfway, it is possible to set the month, date and year without changing the time. The second setting of the crown--all the way out--adjusts the time and the day of the week. A combination of these ...