Yes you can flash with flashtool.
You can use FTF for India. My phone is with same region like yours and I did already update to Marshmallow using FTF for India. Work perfect, don't worry.
If your phone is rooted, Use White Sound Enhancement. I refers this post http://xdaforums.com/showpost.php?p=67856673&postcount=2 . After you install, Open Settings >> Sound and Notification >> Audio Settings >> Sound Effects >> Check XLoud. Restart your phone. Good luck !!!
Okay so I don't know if many of you will see this, but I've started a petition to fix this Marshmallow update permanently. I seriously can't believe the poor choices Sony make with their phones. This Marshmallow update was rushed in the end, and this phone has the potential to be better. Heating issue is kind of resolved, it doesn't heat up as it used to, but there is still a problem with the animations. Stamina mode is really bad, and no adoptable storage. If you want to make a change, then please sign the petition right here: https://www.change.org/p/sony-marshm...xperia-m4-aqua
My model is 2303 and i have locked bootloader im from macedonia what firmwarw should i flash ?
Do you have the ftf for the E2353? That's the only version we get in Australia and the update hasn't reached us yet officially.