Making the jetway go to door 2L instead of 1L is simply a matter of editing the aircraft.cfg file, which is located in the ..\SimObjects\Airplanes\PMDG747-400\ folder. Make a backup copy of the file before doing anything to it. Use Notepad to open the aircraft.cfg file, then find the [exits] section. You should see the specifications for two gates there...
[exits]
number_of_exits = 2
exit.0 = 0.4, -31.1147, -9.5702, 1.11891, 0 //openclose rate percent per second, longitudinal, lateral, vertical positions from datum (feet), type (0=Main 1=Cargo 2=Emergency)
exit.1 = 0.4, -62.2539, -9.6109, 1.1891, 0 //openclose rate percent per second, longitudinal, lateral, vertical positions from datum (feet), type (0=Main 1=Cargo 2=Emergency)
The one labeled exit.0 is always the door to which the jetway attaches. Right now, that is door 1L. To change that, exchange the exit numbers so that they look like this...
exit.1 = 0.4, -31.1147, -9.5702, 1.11891, 0 //openclose rate percent per second, longitudinal, lateral, vertical positions from datum (feet), type (0=Main 1=Cargo 2=Emergency)
exit.0 = 0.4, -62.2539, -9.6109, 1.1891, 0 //openclose rate percent per second, longitudinal, lateral, vertical positions from datum (feet), type (0=Main 1=Cargo 2=Emergency)
That's all it takes. If you want to rearrange the lines so that exit.0 is listed first, that's fine. But the order doesn't matter to the computer — exit.0 is always the jetway door. Be warned that using door 2L often causes the jetway to extend so far that gaps will appear in it. That's an FSX problem for which there is no simple solution.