You can probably change it, but it won’t make much difference. UNcheck the box for “same as source” and play with the settings below to modify your presets. You can change passes under Bit Rate. You can then save it as a new preset if you choose to do so.
In PP CS5.5, I export in the default Youtube Widescreen HD setting all the time. I originally wanted to play around with it but the standard settings did such a good job overall that I haven’t bothered. However, I use AVCHD H.264 codec and perhaps it’s different with that (it’s 1920 x 1080). I checked, and my settings (by default) are 2 pass. My standard settings for Youtube Widescreen SD are 1 pass.
I think it might make SOME difference, depending on the type of source footage you have. But, it will not make nearly as much difference as shooting better source footage like birdcat says.
