iPhone
My iPhone 4 was stolen from me can I still change my passcode ?
Change what password?
You can log in to iCloud.com and attempt to erase the iPhone from there if you had Find My iPhone turned on and it can be found.
You can change your Apple ID password from http://appleid.apple.com but that wil do nothing to the iPhone itself.
If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support