Simply u can't.

agree bro

Hey hoW u ever think that my frd...FlymE is even noT able to Give a staBle update,how it'll give marshmallow Update...I must say-it's a joke n

Flyme a custom firmware based on Android. So directly not possible to upgrade base android version. Also, i don't think Meizu m2 get android m.

for meizu users ,you cant update android instead you can update your firmware of flyme to newer version.If you root the phone then you have n number of android os.make sure you dont brick the phone when installing a custom ROM.Rooting will be safe ,if you have bit tech knowledge about android.If you are not sure I recommend you not to root the phone.If you think your meizu device is just experimental device ,then its your wish.After rooting you cant claim warranty and all is under your own risk.

pls tell me how to upgrade android os

Then try googling XDA forum, or leave it to meizu.