LG L21G Destiny firmware update to Marshmallow?

How can I update my LG L21G Destiny to Marshmallow from Lollipop? The device support section on AT&T's site does not list this phone, but they are my carrier.

The worst thing about Lollipop is that apps I move to the SD card don't stay there when they get updated, and the phone just can't hold very much.

Any help would be appreciated.