Hello! so i was messing around with the FMC and i am flying smootly at FL350 selected a dep approche for landing. but i regret it and want another approce but whenever i select a approche nothing happnes when i go to the legs, it automaticly go to RTA 2 and not the RTA 1 where my route is, i cant even change my Crusing altitude in VNAV page whats happening?

I would really recommend looking into the manuals (and the introduction flight tut.) that PMDG ships with the 777 via the PMDG's plane manager that is installed with the aircraft. You can search for the "FMC operations" part and go through it..