No, there is no way to upgrade the i537 to marshmallow. STOP POSTING THE SAME QUESTION TO 10 DIFFERENT THREADS. You have no idea how irritating that is. If you had bothered using the search function, you might've come across this useful post (I found it in 10 seconds): http://forum.xda-developers.com/show...&postcount=573
Edit: REALLY??!! You're posting this question to Galaxy S5 threads now, how could you possibly think that would get the answer you're looking for??
And you posted this dumb cookie cutter post to a Galaxy S3 forum thread, dude take the time to learn how to use this forum correctly, this is juvenile behavior.
To elaborate, the i537 cannot be upgraded to marshmallow because there has been no official marshmallow release for this specific phone (and almost certainly never will be). This means a custom rom with a custom kernel would be required to run marshmallow, but the bootloader is locked down on this phone and unlockable. A locked bootloader means you simply cannot install a custom kernel on this phone, it will not boot if you try. Only custom roms that can run on the stock kernel can work on this phone; marshmallow will not run on the stock 5.0.1 (lollipop) kernel, therefore this phone cannot run marshmallow. This is why locked bootloaders suck
Sorry for the reposting, just needed some answers, I'm amateur