Usually this is done through the carrier but your change may have dropped that path.
Not all phones get this update so I take it you found it on other same models.
I bought a Samsung Galaxy S6 Edge from T-Mobile and after a month I dropped the account with them and join Straight Talk, although it is still a T-Mobile phone, it is working fine, but I can't find a way to upgrade it to Marshmallow. Any help?

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